Failure support for libcore

The core library cannot define failure, but it does declare failure. This means that the functions inside of libcore are allowed to fail, but to be useful an upstream crate must define failure for libcore to use. The current interface for failure is:

fn begin_unwind(fmt: &fmt::Arguments, file: &str, line: uint) -> !;

This definition allows for failing with any general message, but it does not allow for failing with a ~Any value. The reason for this is that libcore is not allowed to allocate.

This module contains a few other failure functions, but these are just the necessary lang items for the compiler. All failure is funneled through this one function. Currently, the actual symbol is declared in the standard library, but the location of this may change over time.