Update documentation for from_raw_parts

Fixes #19269.
librustc: Don't reuse same alloca for match on struct/tuple field which we reassign to in match body.
collections: dramatically speed up Vec::reserve with magic

Somehow llvm is able to optimize this version of Vec::reserve
into dramatically faster than the old version. In micro-benchmarks
this was 2-10 times faster. It also shaved 14 minutes off of
rust's compile times.

Closes #19281.
Add doctests for keys() and values() of the BTreeMap collection.

Update keys() and values() for BTreeMap based on @Gankro's comments.

Assign keys and values to variables before doing assertion.
Implement BitOps for TreeSet
Allow message specification for should_fail

The test harness will make sure that the panic message contains the
specified string. This is useful to help make `#[should_fail]` tests a
bit less brittle by decreasing the chance that the test isn't
"accidentally" passing due to a panic occurring earlier than expected.
The behavior is in some ways similar to JUnit's `expected` feature:
`@Test(expected=NullPointerException.class)`.

Without the message assertion, this test would pass even though it's not
actually reaching the intended part of the code:
```rust
 #[test]
 #[should_fail(message = "out of bounds")]
fn test_oob_array_access() {
    let idx: uint = from_str("13o").unwrap(); // oops, this will panic
    [1i32, 2, 3][idx];
}
```
Change from message to expected
libcollections: remove unnecessary `as_slice()` calls
liballoc: remove unnecessary `as_slice()` calls
libcoretest: remove unnecessary `as_slice()` calls
libflate: remove unnecessary `as_slice()` calls
libfmt_macros: remove unnecessary `as_slice()` calls
libgetops: remove unnecessary `as_slice()` calls
libgraphviz: remove unnecessary `as_slice()` calls
liblog: remove unnecessary `as_slice` calls
libregex: remove unnecessary `as_slice` calls
libregex_macros: remove unnecessary `as_slice` calls
librustc: remove unnecessary `as_slice()` calls
librustc_back: remove unnecessary `as_slice()` calls
librustc_trans: remove unnecessary `as_slice` calls