Fix target-feature codegen option syntax in rustc man page.

The man page stated that the list of features was space-separated when
it's actually comma-separated.
adding iterator benches to tree-based maps for comparison purposes
flesh out BTree docs
doc fixups
vec: minor cleanup
auto merge of #17793 : simias/rust/master, r=huonw

The man page stated that the list of features was space-separated when
it's actually comma-separated.
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.
compiletest needs to link to native crate, or at least the `rt` library.

(I tried using a dependency on `rustrt` instead, and that did not
resolve the problem.  But this does.)

Fix #17883
auto merge of #17940 : pnkfelix/rust/fix-check-stage1-on-clean-dir, r=huonw

compiletest needs to link to native crate, or at least the `rt` library.

(I tried using a dependency on `rustrt` instead, and that did not resolve the problem.  But this does.)

Partially addresses #17883
auto merge of #17801 : Gankro/rust/collections-stuff, r=sfackler

I previously avoided `#[inline]`ing anything assuming someone would come in and explain to me where this would be appropriate. Apparently no one *really* knows, so I'll just go the opposite way an inline everything assuming someone will come in and yell at me that such-and-such shouldn't be `#[inline]`.

==================

For posterity, iteration comparisons:

```
test btree::map::bench::iter_20                            ... bench:       971 ns/iter (+/- 30)
test btree::map::bench::iter_1000                          ... bench:     29445 ns/iter (+/- 480)
test btree::map::bench::iter_100000                        ... bench:   2929035 ns/iter (+/- 21551)

test treemap::bench::iter_20                               ... bench:       530 ns/iter (+/- 66)
test treemap::bench::iter_1000                             ... bench:     26287 ns/iter (+/- 825)
test treemap::bench::iter_100000                           ... bench:   7650084 ns/iter (+/- 356711)

test trie::bench_map::iter_20                              ... bench:       646 ns/iter (+/- 265)
test trie::bench_map::iter_1000                            ... bench:     43556 ns/iter (+/- 5014)
test trie::bench_map::iter_100000                          ... bench:  12988002 ns/iter (+/- 139676)
```

As you can see `btree` "scales" much better than `treemap`. `triemap` scales quite poorly.

Note that *completely* different results are given if the elements are inserted in order from the range [0, size]. In particular, TrieMap *completely* dominates in the sorted case. This suggests adding benches for both might be worthwhile. However unsorted is *probably* the more "normal" case, so I consider this "good enough" for now.