Unwinding support

[Amanieu]

See previous discussion in #52.

[Amanieu]

One unresolved question is what to do about unfinished generators in the following cases with & without unwinding:

• unwrap
• unsafe_unwrap
• drop

[carllerche]

Is this PR planned to be merged?

[edef1c]

@carllerche Yes, the main blocker is uncertainty about how this can be made to work well with -C panic=abort.

[carllerche]

Ok great. I would like to use fringe here: http://github.com/carllerche/loom

Right now I’m using a fork that includes this PR. It would also be great if fringe could work on stable :-)

[edef1c]

That's an exciting project! I'm afraid that getting libfringe to work on stable Rust would require stabilising inline assembly, though there are alternative (slower) approaches that could be implemented with similar APIs.

[carllerche]

@edef1c It would be nice if there were a stable (slower) option w/ a nightly feature flag. Right now, libfringe is the only Rust generator lib that does the right thing. generator-rs seems to SEGV if RUST_BACKTRACE=1 is specified.