Make `Vec::new` a `const fn`

`RawVec::empty/_in` are a hack. They're there because `if size_of::<T> == 0 { !0 } else { 0 }` is not allowed in `const` yet. However, because `RawVec` is unstable, the `empty/empty_in` constructors can be removed when rust-lang#49146 is done...

Add more links in panic docs

Fixes rust-lang#48695 by adding a link to `AssertUnwindSafe`. Also added some other links in the module's docs to make things clearer.

Fix some broken links in docs.

Add a few more tests for proc macro feature gating

…=estebank

Display correct unused field suggestion for nested struct patterns

Extends rust-lang#47922 by checking more sophisticated patterns (e.g. references, slices, etc.).
Before:
```
warning: unused variable: `bar`
  --> src/main.rs:37:21
   |
37 |         &Foo::Bar { bar } => true,
   |                     ^^^ help: consider using `_bar` instead
   |
   = note: #[warn(unused_variables)] on by default
```
After:
```
warning: unused variable: `bar`
  --> src/main.rs:37:21
   |
37 |         &Foo::Bar { bar } => true,
   |                     ^^^ help: try ignoring the field: `bar: _`
   |
   = note: #[warn(unused_variables)] on by default
```

Fixes rust-lang#50303.

r? @estebank

check that #[used] is used only on statics

this attribute has no effect on other items. This makes the implementation match what's described in the RFC.

cc rust-lang#40289

r? @nagisa

Update Cargo to 2018-04-28 122fd5be5201913d42e219e132d6569493583bca