Allow mutable slices in statics.

Fixes #11411
rustc: Move mut slice check to `check_static`

This is a follow-up patch that moves the mut slice check to the recently
added `check_static`

Closes #11411
num: Slightly optimize bigint
num: Migrate `~[T]` to `std::vec_ng::Vec`
mk: Clean libbacktrace w/ gusto

After `make clean' I'm seeing the build break with

```
cp: cannot stat ‘x86_64-unknown-linux-gnu/rt/libbacktrace/.libs/libbacktrace.a’: No such file or directory
```

Deleteing the libbacktrace dir entirely on clean fixes.
Remove std::cmp::cmp2.

It isn't used anywhere and `cmp2(a, b, c, d)` is identical to
`(a, b).cmp(&(c, d))`.
Tag derived impls with #[automatically_derived]

This will enable rustdoc to treat them specially.
auto merge of #12899 : brson/rust/cleanbacktrace, r=alexcrichton

After `make clean` I'm seeing the build break with

```
cp: cannot stat ‘x86_64-unknown-linux-gnu/rt/libbacktrace/.libs/libbacktrace.a’: No such file or directory
```

Deleteing the libbacktrace dir entirely on clean fixes.
auto merge of #12929 : sfackler/rust/automatically-derived, r=cmr

This will enable rustdoc to treat them specially.

I also got rid of `std::cmp::cmp2`, which is isomorphic to the `TotalOrd` impl for 2-tuples and never used.
Refactored take_unwrap (libstd/option.rs)

Using pattern matching instead of is_some + unwrap
auto merge of #12924 : Florob/rust/bigint, r=alexcrichton

This is a minor optimization of the bignum module. The improvements mostly come from avoiding allocations and boundary checks. This also switches all of libnum to vec_ng::Vec.
Fix a test that was missed in the liblog PR
auto merge of #12933 : edwardw/rust/fallout, r=huonw

The same test was missed in chan/port renaming PR #12815 and was fixed in #12880:

> This was missed because it is skipped on linux and windows, and the mac bots were moving at the time the PR landed.

It seems the same happened to the liblog PR.
Asked if missing (), then asks about an anonymous function. Also added test.
getopts missing docs
auto merge of #12931 : aochagavia/rust/option-take_unwrap, r=cmr

Using pattern matching instead of is_some + unwrap
Add method Vec<T>::as_mut_ptr()
Document the Tm struct and fields
Remove AtomicFlag

fixes #12943
Doc-sprint: Document endian conversion functions