demodeing, un-gc-ing

It looks to me like the string_reader and tt_reader structs are
GC pointers only because they predate the modern borrow system.
This commit leaves the type names string_reader and tt_reader alone
(they still refer to GC-ed pointers), but internally the functions
now use borrowed pointers to refer to these structures. My guess
would be that it's possible to move this change outward and not
use the GCed pointers at all, but that change looks like it could be
a larger one. Actually, I'm delighted at how quick this change was.
lines too long
core: Add a rand::Rand trait that is implemented by types that can be
randomly generated
Also adds Rng::gen() for generating any type that implements the Rand
trait
Fix some bugs with -W unused-imports

1. Don't warn about anything not used in the prelude which is autmoatically
   injected, accomplished with a test that the span is equal to a dummy span.
2. Don't warn about unused imports from the injected intrinsic module,
   accomplished by testing against the name of the imported module
3. If anything is used from a glob import, don't warn about the glob import.
4. If an import imports more than one thing, and none of them are used, only
   issue a warning once

Also updated the unused-imports-warn test to have stricter requirements on
error messages.
Don't warn when imported traits are indeed used
Merge branch 'incoming' into made-snap
Merge branch 'incoming' into removing
Revert "oldmap: use &K instead of K in find and get"

This reverts commit 8e643525d4e5bca993dada43615916c382a0645b.
testsuite: Add test for #4523
oldmap: use &K instead of K in find and get

This reverts commit a4250a96fdf61142a9c8dbb6d37ae8435c99e396.

This is not the cause of the nonexhaustive-match failure.
mk: Parameterize tests.mk to the max
mk: Don't check core/std/rustc as part of check-fast

When debugging check-fast it's annoying to wait through core/std
libsyntax: no binary/hex float literals
got rid of last empty lines
`core::send_map` renamed to `core::hashmap`

Encountered this while trying to port rustsqlite to rust 0.6
mk: Add output files to tests so they don't rerun unnecessarily
rt: Add rust_try_get_current_task
rustc: Less copy
Make foreign calls work outside of tasks. #4451
Rewrite the exchange allocator to work without an active scheduler. #4457