Change to_str().to_string() to just to_str()
Fix documentation for `slice()`

Fixes https://github.com/mozilla/rust/issues/14577
Document BigInt's new and from_slice methods

Fixes https://github.com/mozilla/rust/issues/14639
Update AUTHORS.txt
Remove a warning  (./configure: 1140: [: unexpected operator) when built with --disable-manage-submodules
Implement #[plugin_registrar]
Use phase(plugin) in bootstrap crates

Do this to avoid warnings on post-stage0 builds.
Use phase(plugin) in other crates
Use phase(plugin) in tests
Convert libraries to use #[plugin_registrar]
Convert tests to use #[plugin_registrar]
Add a test for deprecated phase(syntax)
Document rustc::plugin
Macro crates now depend on librustc