Fixes#658
This commit adds support for unwinding after a panic. It requires a
companion rustc PR to be merged, in order for the necessary hooks to
work properly.
Currently implemented:
* Selecting between unwind/abort mode based on the rustc Session
* Properly popping off stack frames, unwinding back the caller
* Running 'unwind' blocks in Mir terminators
Not yet implemented:
* 'Abort' terminators
This PR was getting fairly large, so I decided to open it for review without
implementing 'Abort' terminator support. This could either be added on
to this PR, or merged separately.
