Updated to a recent nightly Rust

Builds with rustc 0.13.0-nightly (2274996a1 2014-11-23 00:36:43 +0000)
Pulled native into mainloop

There is no runtime system in the standard library now (see
[Rust issue #17325] (https://github.com/rust-lang/rust/issues/17325)),
so the original separation of code specific to the native runtime
has lost its purpose.
Define the gboolean constants with const