This improves -O0 compile times dramatically.

This can perhaps be restored in the future. For now this is a precursor to
making typedefs work as expected.

Implement SmallIntSet (refer #4984)

Fix issue #6209, and some related issues about constant expression
 - unmatched type between arms
 - unmatched type in range

The error message is extremely unideal.

Fix #6342

This verifies that a static item evaluates to true, at compile time.

This verifies that a static item evaluates to true, at compile time.

This fixes segmentation fault of new rt tests.
For example
```
use core::rt::test::*;
use core::rt::comm::*;
use core::cell::Cell;

fn main() {
    do run_in_newsched_task {
        let (port, chan) = oneshot::<~int>();
        let port_cell = Cell(port);
        do spawntask_immediately {
            assert!(port_cell.take().recv() == ~10);
        }
        chan.send(~10);
    }
}
```

r? @nikomatsakis The way we deal with unreachable expressions in trans is pretty ad hoc,
but this at least doesn't make it worse, and eliminates the LLVM
assertion failure reported in #5741.

The way we deal with unreachable expressions in trans is pretty ad hoc,
but this at least doesn't make it worse, and eliminates the LLVM
assertion failure reported in #5741.

Fixes https://github.com/mozilla/rust/issues/6578 by merging the 3 different ways to build an AST into a single `AstBuilder` trait, creating a more uniform and briefer interface.

Also, converts the `ext_ctxt` trait-object to be a plain struct, as well as renaming it to `ExtCtxt`.

Seems to make expansion slightly faster for the normal case (e.g. `libcore` and `libstd`), but slower for `librustc` (slightly) and `libsyntax` (0.3s -> 0.8s! I'm investigating this, but I'd prefer this patch to land relatively quickly.).

`git blame` suggests maybe @graydon or @erickt are familiar with this area of the code. r?