vec: minor cleanup
implement Box<[T]> <-> Vec<T> conversions
Remove the runtime guide.

Now that libgreen is gone, this is all wrong.

Fixes #17923
improve the performance of the vec![] macro

Closes #17865
auto merge of #17920 : thestinger/rust/vec, r=aturon

Introduce conversions between `Box<[T]>` and `Vec<T>` and use it to reimplement the `vec![]` macro for efficiency.
remove shootout warnings
auto merge of #17936 : TeXitoi/rust/remove-shootout-warnings, r=alexcrichton

Only one warning remain, and I can't find a way to remove it without doing more bound checks:

```
shootout-nbody.rs:105:36: 105:51 warning: use of deprecated item: use iter_mut, #[warn(deprecated)] on by default
shootout-nbody.rs:105             let bi = match b_slice.mut_shift_ref() {
```

using `split_at_mut` may be an option, but it will do more bound checking.

If anyone have an idea, I'll update this PR.
Register new snapshots

Also convert a number of `static mut` to just a plain old `static` and remove
some unsafe blocks.
auto merge of #17928 : steveklabnik/rust/remove_runtime_guide, r=alexcrichton

Now that libgreen is gone, this is all wrong.

Fixes #17923
auto merge of #17939 : alexcrichton/rust/snapshots, r=sfackler

Also convert a number of `static mut` to just a plain old `static` and remove
some unsafe blocks.