std: Remove ThreadPerCore spawn mode. Unused
rustc compiler config for Vim.

Note that this is not actually *used* by default; it is a matter of
configuration still, because you might want to:

- Compile all .rs files with `rustc %` (where each can be built itself)

- Compile all .rs files with `rustc some-file.rs` (where you are editing
  part of a crate)

- Compile with a different tool, such as `make`. (In this case you might
  put a `~/.vim/after/compiler/rustc.vim` to match such cases, set
  makeprg and extend errorformat as appropriate. That should probably go
  in a different compiler mode, e.g. make-rustc.)

To try using it, `:compiler rustc`. Then, `:make` on a file you would
run `rustc` on will work its magic, invoking rustc. To automate this,
you could have something like `autocmd FileType rust compiler rustc` in
your Vim config.
Provide more useful messages when tests are given -h or --help

Progress on #7824
tutorial: remove duplicate link references.

`pandoc` issues warnings.

```
pandoc: Duplicate link reference `[macros]' "source" (line 2151, column 1)
pandoc: Duplicate link reference `[tasks]' "source" (line 2150, column 1)
pandoc: Duplicate link reference `[wiki-start]' "source" (line 92, column 1)
```
tutorial: remove unused link references.
Remove --quiet from git submodule operations
Add documentation about mutable statics to rust.md
stop initializing ref_count for stack closures

the only part of the header ~fn and &fn should be touching right now is
the tydesc
rm unused visit_str method from TyVisitor
std::rt: Rename RUST_TEST_THREADS to RUST_RT_TEST_THREADS
rm unnecessary stage0 `zero_memory` fn
rt: Diagnose bad alloc index in release_alloc with RUSTRT_TRACK_ALLOCATIONS
rt: Print alloc backtraces for bad release_alloc with RUSTRT_TRACK_ALLOCATIONS

Probably the env var should be renamed from DETAILED_LEAKS but I'm leaving
aside that breaking change for now.
testsuite: Add compile-fail test for #5883
fix repr of unique vectors with stage0 libstd

Closes #7860
testsuite: Test for #6153, closes #6153
Fix warnings in libextra tests

Most of these are "unneccesary allocation" in bitv, for ~[false, ..] instead
of [false, ..].
Fix warnings in libstd and librusti tests
remove non-portable sed invocation as brson says we no longer have clang in tree.
librustc: Remove a bunch of `@` boxes from `Match`.

This will be needed to add `'static` bounds to `@`.