Co-Authored-By: Ashley Mannix <ashleymannix@live.com.au>

…itions

Co-Authored-By: Mark Rousskov <mark.simulacrum@gmail.com>

Implement clone_from for BTreeMap and BTreeSet

See rust-lang#28481. This results in up to 90% speedups on simple data types when `self` and `other` are the same size, and is generally comparable or faster. Some concerns:

1. This implementation requires an `Ord` bound on the `Clone` implementation for `BTreeMap` and `BTreeSet`. Since these structs can only be created externally for keys with `Ord` implemented, this should be fine? If not, there's certainly a less safe way to do this.
2. Changing `next_unchecked` on `RangeMut` to return mutable key references allows for replacing the entire overlapping portion of both maps without changing the external interface in any way. However, if `clone_from` fails it can leave the `BTreeMap` in an invalid state, which might be unacceptable.

~This probably needs an FCP since it changes a trait bound, but (as far as I know?) that change cannot break any external code.~

…ark-Simulacrum

BTreeMap: tag and explain unsafe internal functions or assert preconditions

rust-lang#68418 concluded that it's not desirable to tag all internal functions with preconditions as being unsafe. This PR does it to some functions, documents why, and elsewhere enforces the preconditions with asserts.

Use termize instead of term_size

`termize` is a fork of `term_size` which uses `winapi` 0.3 instead of 0.2. This is a step towards removing the `winapi` 0.2 dependency.

r? @Mark-Simulacrum

…eklabnik

Document remaining undocumented `From` implementations for IPs

Some `From` implementations were missing documentation. I added examples, I tried to be as close to existing examples as possible.

Related to rust-lang#51430 (cc @skade)

r? @steveklabnik

…=steveklabnik

Document `From` implementation for NonZero nums

This is more of a nitpick than a real change in documentation. I did this for consistency with other documentation (namely Atomic integers).

Related to rust-lang#51430

r? @steveklabnik

PS: I actually tested the code this time. My CPU died in the process, but I get to open a (hopefully 🤞) working PR

Fix revision annotations in borrowck-feature-nll-overrides-migrate