Remove unused imports

I was able to remove unused imports, and fix the following warnings

src/libstd/hashmap.rs:23:15: 23:23 warning: unused import [-W unused-imports (default)]
src/libstd/task/spawn.rs:95:15: 95:23 warning: unused import [-W unused-imports (default)]
src/libstd/rt/uv/mod.rs:42:0: 42:9 warning: unused import [-W unused-imports (default)]
src/libstd/rt/uv/mod.rs:45:0: 45:9 warning: unused import [-W unused-imports (default)]
src/librustc/middle/trans/meth.rs:26:0: 26:26 warning: unused import [-W unused-imports (default)]
src/librustc/back/link.rs:210:20: 210:25 warning: unused import [-W unused-imports (default)]

I was unable to fix the following unused import warnings. The code
here was weird.

src/libextra/std.rc:40:11: 40:14 warning: unused import [-W unused-imports (default)]
src/libextra/std.rc:40:16: 40:24 warning: unused import [-W unused-imports (default)]
auto merge of #7105 : sstewartgallus/rust/removed_unused_imports, r=sanxiyn

I was able to remove unused imports, and fix the following warnings

src/libstd/hashmap.rs:23:15: 23:23 warning: unused import [-W unused-imports (default)]
src/libstd/task/spawn.rs:95:15: 95:23 warning: unused import [-W unused-imports (default)]
src/libstd/rt/uv/mod.rs:42:0: 42:9 warning: unused import [-W unused-imports (default)]
src/libstd/rt/uv/mod.rs:45:0: 45:9 warning: unused import [-W unused-imports (default)]
src/librustc/middle/trans/meth.rs:26:0: 26:26 warning: unused import [-W unused-imports (default)]
src/librustc/back/link.rs:210:20: 210:25 warning: unused import [-W unused-imports (default)]

I was unable to fix the following unused import warnings. The code
here was weird.

src/libextra/std.rc:40:11: 40:14 warning: unused import [-W unused-imports (default)]
src/libextra/std.rc:40:16: 40:24 warning: unused import [-W unused-imports (default)]
Revert "Have JIT execution take ownership of the LLVMContextRef"

This reverts commit 5c5095d25e3652c434c8d4ec178e6844877e3c2d.

Conflicts:
	src/librusti/rusti.rc
Revert "Remove all usage of the global LLVMContextRef"

This reverts commit 779191cd4b8719e8efdf69fb6da93e2a8905ca1d.

Conflicts:
	src/librustc/middle/trans/base.rs
	src/librustc/middle/trans/common.rs
Update CONTRIBUTING.md
syntax: correct the Rand::rand call to select enum variants in #[deriving(Rand)].

Previously, this was not a global call, and so when `#[deriving(Rand)]`
was in any module other than the top-level one, it failed (unless there
was a `use std;` in scope).

Also, fix a minor inconsistency between uints and u32s for this piece
of code.
syntax: revert the uint -> u32 "fix"; make the names/comment match.
xfail test on windows, because extern mod doesn't work there