This adds an small of failure to libcore, hamstrung by the fact that std::fmt
hasn't been migrated yet. A few asserts were re-worked to not use std::fmt
features, but these asserts can go back to their original form once std::fmt has
migrated.
The current failure implementation is to just have some symbols exposed by
std::rt::unwind that are linked against by libcore. This is an explicit circular
dependency, unfortunately. This will be officially supported in the future
through compiler support with much nicer failure messages. Additionally, there
are two depended-upon symbols today, but in the future there will only be one
(once std::fmt has migrated).
A task without an unwinder will abort the process on failure.
I'm using this in the runtime tests to guarantee that a call to
`assert!` actually triggers some kind of failure (an abort)
instead of silently doing nothing. This is essentially in lieu
of a working linked failure implementation.
