sync: Move underneath libstd

This commit is the final step in the libstd facade, #13851. The purpose of this
commit is to move libsync underneath the standard library, behind the facade.
This will allow core primitives like channels, queues, and atomics to all live
in the same location.

There were a few notable changes and a few breaking changes as part of this
movement:

* The `Vec` and `String` types are reexported at the top level of libcollections
* The `unreachable!()` macro was copied to libcore
* The `std::rt::thread` module was moved to librustrt, but it is still
  reexported at the same location.
* The `std::comm` module was moved to libsync
* The `sync::comm` module was moved under `sync::comm`, and renamed to `duplex`.
  It is now a private module with types/functions being reexported under
  `sync::comm`. This is a breaking change for any existing users of duplex
  streams.
* All concurrent queues/deques were moved directly under libsync. They are also
  all marked with #![experimental] for now if they are public.
* The `task_pool` and `future` modules no longer live in libsync, but rather
  live under `std::sync`. They will forever live at this location, but they may
  move to libsync if the `std::task` module moves as well.

[breaking-change]

mk/crates.mk

@@ -61,16 +61,16 @@ DEPS_rlibc :=
 DEPS_alloc := core libc native:jemalloc
 DEPS_debug := std
 DEPS_rustrt := alloc core libc collections native:rustrt_native
-DEPS_std := core libc rand alloc collections rustrt \
+DEPS_std := core libc rand alloc collections rustrt sync \
 	native:rust_builtin native:backtrace
 DEPS_graphviz := std
 DEPS_green := std native:context_switch
 DEPS_rustuv := std native:uv native:uv_support
 DEPS_native := std
 DEPS_syntax := std term serialize log fmt_macros debug
-DEPS_rustc := syntax native:rustllvm flate arena serialize sync getopts \
+DEPS_rustc := syntax native:rustllvm flate arena serialize getopts \
               time log graphviz debug
-DEPS_rustdoc := rustc native:hoedown serialize sync getopts \
+DEPS_rustdoc := rustc native:hoedown serialize getopts \
                 test time debug
 DEPS_flate := std native:miniz
 DEPS_arena := std
@@ -80,17 +80,17 @@ DEPS_serialize := std log
 DEPS_term := std log
 DEPS_semver := std
 DEPS_uuid := std serialize
-DEPS_sync := std alloc
+DEPS_sync := core alloc rustrt collections
 DEPS_getopts := std
 DEPS_collections := core alloc
 DEPS_fourcc := rustc syntax std
 DEPS_hexfloat := rustc syntax std
 DEPS_num := std
 DEPS_test := std getopts serialize term time regex native:rust_test_helpers
-DEPS_time := std serialize sync
+DEPS_time := std serialize
 DEPS_rand := core
 DEPS_url := std
-DEPS_log := std sync
+DEPS_log := std
 DEPS_regex := std
 DEPS_regex_macros = rustc syntax std regex
 DEPS_fmt_macros = std