Results of libcore and libstd tests

```
failures:
    rand::tests::test_rng_seeded_custom_seed2
    time::tests::run_tests
    uv_ll::test::test_uv_ll_struct_size_addrinfo
    uv_ll::test::test_uv_ll_struct_size_uv_timer_t

segfaults:
    stackwalk::test_simple
    stackwalk::test_simple_deep
```

Resolves #6633 and removes an unnecessary import in libcore

This adds a lint mode for detecting unnecessary allocations on the heap. This isn't super fancy, currently it only has two rules

1. For a function's arguments, if you allocate a `[~|@]str` literal, when the type of the argument is a `&str`, emit a warning.
2. For the same case, emit warnings for boxed vectors when slices are required.

After adding the lint, I rampaged through the libraries and removed all the unnecessary allocations I could find.

r?

Mostly refactoring, and adding some of the remaining types described in #4419.

The [`Local`](https://github.com/brson/rust/blob/3b4ff41511cfaa5e311b03d16b47bf40c117fa2f/src/libcore/rt/local.rs#L17) trait collects some common, often unsafe patterns around task-local and thread-local values. Making all these types safe is largely the aim of #6210.

r? @pcwalton

Remove the uncommon term 'object tree' in favor of 'value'.

...s

This is the queue used to send messages to Schedulers

* Add ARC::get method and implements the function from it.
* Add an example showing a simple use of ARC.

Update PR #6622 to avoid git noise.

I will remove the function get later.

This includes new, tested, hygiene support functions. It also removes the interner_key! macro and replaces it with a function, which should be inline-able. It also contains some parser patch-ups and some docfixes.

On my machine, this patch passes all tests.