std: Read HOME instead of USER

Apparently one of the linux bots doesn't have the USER variable defined, and
this fix will likely land more quickly than a fix to the bots.
auto merge of #14775 : alexcrichton/rust/fix-master, r=brson

Apparently one of the linux bots doesn't have the USER variable defined, and
this fix will likely land more quickly than a fix to the bots.
Implement #[plugin_registrar]

See RFC 22.

[breaking-change]
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
auto merge of #14554 : kmcallister/rust/plugin_registrar, r=cmr

This implements the design in rust-lang/rfcs#86.  It shouldn't be merged until that RFC is accepted, but it would be great if somebody has time to review the code before then.
std: Remove the as_utf16_p functions

These functions are all much better expressed via RAII using the to_utf16()
method on strings. This refactoring also takes this opportunity to properly
handle when filenames aren't valid unicode when passed through to the windows
I/O layer by properly returning I/O errors.

All previous users of the `as_utf16_p` or `as_utf16_mut_p` functions will need
to convert their code to using `foo.to_utf16().append_one(0)` to get a
null-terminated utf16 string.

[breaking-change]
Moved integer trait and functions to submodule
Cleanup bigint
Add a --color flag to test binaries

It uses the same behavior as rustc's.
std: Move dynamic_lib from std::unstable to std

This leaves a deprecated reexport in place temporarily.

Closes #1457.
Add test for issue #13446

Closes #13446
collections: Add missing Default impls

Add Default impls for TreeMap, TreeSet, SmallIntMap, BitvSet, DList,
PriorityQueue, RingBuf, TrieMap, and TrieSet.
native: add more info to the native unimplemented error.

This refers to green, which (AFAICT) has everything implemented. In
particular, this will help guide people to get working signal handling
via libgreen.