Stabilize nested self receivers in 1.41.0

Previously, only `Self`, `&Self`, `&mut Self`, `Arc<Self>`, `Rc<Self>`,
and `Box<Self>` were available as stable method receivers.

This commit stabilizes nested uses of all the above types.
However, nested receivers remain non-object-safe.

Rollup of 17 pull requests

Successful merges:

 - #64325 (Stabilize nested self receivers in 1.41.0)
 - #66222 (Use `eq_opaque_type_and_type` when type-checking closure signatures)
 - #66305 (Add by-value arrays to `improper_ctypes` lint)
 - #66399 (rustc_metadata: simplify the interactions between Lazy and Table.)
 - #66534 (Allow global references via ForeignItem and Item for the same symbol name during LLVM codegen)
 - #66700 (Fix pointing at arg for fulfillment errors in function calls)
 - #66704 (Intra doc enum variant field)
 - #66718 (Refactor `parse_enum_item` to use `parse_delim_comma_seq`)
 - #66722 (Handle non_exhaustive in borrow checking)
 - #66744 (Fix shrink_to panic documentation)
 - #66761 (Use LLVMDisposePassManager instead of raw delete in rustllvm)
 - #66769 (Add core::{f32,f64}::consts::TAU.)
 - #66774 (Clean up error codes)
 - #66777 (Put back tidy check on error codes)
 - #66797 (Fixes small typo in array docs r? @steveklabnik)
 - #66798 (Fix spelling typos)
 - #66800 (Combine similar tests for const match)

Failed merges:

r? @ghost

Version 1.41.0 (2020-01-30)
===========================

Language
--------

- [You can now pass type parameters to foreign items when implementing
  traits.][65879] E.g. You can now write `impl<T> From<Foo> for Vec<T> {}`.
- [You can now arbitrarily nest receiver types in the `self` position.][64325] E.g. you can
  now write `fn foo(self: Box<Box<Self>>) {}`. Previously only `Self`, `&Self`,
  `&mut Self`, `Arc<Self>`, `Rc<Self>`, and `Box<Self>` were allowed.
- [You can now use any valid identifier in a `format_args` macro.][66847]
  Previously identifiers starting with an underscore were not allowed.
- [Visibility modifiers (e.g. `pub`) are now syntactically allowed on trait items and
  enum variants.][66183] These are still rejected semantically, but
  can be seen and parsed by procedural macros and conditional compilation.

Compiler
--------

- [Rustc will now warn if you have unused loop `'label`s.][66325]
- [Removed support for the `i686-unknown-dragonfly` target.][67255]
- [Added tier 3 support\* for the `riscv64gc-unknown-linux-gnu` target.][66661]
- [You can now pass an arguments file passing the `@path` syntax
  to rustc.][66172] Note that the format differs somewhat from what is
  found in other tooling; please see [the documentation][argfile-docs] for
  more information.
- [You can now provide `--extern` flag without a path, indicating that it is
  available from the search path or specified with an `-L` flag.][64882]

\* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.

[argfile-docs]: https://doc.rust-lang.org/nightly/rustc/command-line-arguments.html#path-load-command-line-flags-from-a-path

Libraries
---------

- [The `core::panic` module is now stable.][66771] It was already stable
  through `std`.
- [`NonZero*` numerics now implement `From<NonZero*>` if it's a smaller integer
  width.][66277] E.g. `NonZeroU16` now implements `From<NonZeroU8>`.
- [`MaybeUninit<T>` now implements `fmt::Debug`.][65013]

Stabilized APIs
---------------

- [`Result::map_or`]
- [`Result::map_or_else`]
- [`std::rc::Weak::weak_count`]
- [`std::rc::Weak::strong_count`]
- [`std::sync::Weak::weak_count`]
- [`std::sync::Weak::strong_count`]

Cargo
-----

- [Cargo will now document all the private items for binary crates
  by default.][cargo/7593]
- [`cargo-install` will now reinstall the package if it detects that it is out
  of date.][cargo/7560]
- [Cargo.lock now uses a more git friendly format that should help to reduce
  merge conflicts.][cargo/7579]
- [You can now override specific dependencies's build settings][cargo/7591] E.g.
  `[profile.dev.overrides.image] opt-level = 2` sets the `image` crate's
  optimisation level to `2` for debug builds. You can also use
  `[profile.<profile>.build_overrides]` to override build scripts and
  their dependencies.

Misc
----

- [You can now specify `edition` in documentation code blocks to compile the block
  for that edition.][66238] E.g. `edition2018` tells rustdoc that the code sample
  should be compiled the 2018 edition of Rust.
- [You can now provide custom themes to rustdoc with `--theme`, and check the
  current theme with `--check-theme`.][54733]
- [You can use `#[cfg(doc)]` to compile an item when building documentation.][61351]

Compatibility Notes
-------------------

- [As previously announced 1.41.0 will be the last tier 1 release for 32-bit
  Apple targets.][apple-32bit-drop] This means that the source code is still
  available to build, but the targets are no longer being tested and release
  binaries for those platforms will no longer be distributed by the Rust project.
  Please refer to the linked blog post for more information.

[54733]: rust-lang/rust#54733
[61351]: rust-lang/rust#61351
[67255]: rust-lang/rust#67255
[66661]: rust-lang/rust#66661
[66771]: rust-lang/rust#66771
[66847]: rust-lang/rust#66847
[66238]: rust-lang/rust#66238
[66277]: rust-lang/rust#66277
[66325]: rust-lang/rust#66325
[66172]: rust-lang/rust#66172
[66183]: rust-lang/rust#66183
[65879]: rust-lang/rust#65879
[65013]: rust-lang/rust#65013
[64882]: rust-lang/rust#64882
[64325]: rust-lang/rust#64325
[cargo/7560]: rust-lang/cargo#7560
[cargo/7579]: rust-lang/cargo#7579
[cargo/7591]: rust-lang/cargo#7591
[cargo/7593]: rust-lang/cargo#7593
[`Result::map_or_else`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.map_or_else
[`Result::map_or`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.map_or
[`std::rc::Weak::weak_count`]: https://doc.rust-lang.org/std/rc/struct.Weak.html#method.weak_count
[`std::rc::Weak::strong_count`]: https://doc.rust-lang.org/std/rc/struct.Weak.html#method.strong_count
[`std::sync::Weak::weak_count`]: https://doc.rust-lang.org/std/sync/struct.Weak.html#method.weak_count
[`std::sync::Weak::strong_count`]: https://doc.rust-lang.org/std/sync/struct.Weak.html#method.strong_count
[apple-32bit-drop]: https://blog.rust-lang.org/2020/01/03/reducing-support-for-32-bit-apple-targets.html