Make unwinding more efficient

[brson]

We generate much more unwind code than is necessary, the full set of in-scope cleanups for every single function call. This could be more efficient by reusing landing pads and chaining scope unwind cleanup blocks together. Ideally we would have zero or one unwind block per scope.

[brson]

I've done some work on this, so the current situation isn't so dire. Ideally we would have zero or one landing pad per scope, and the landing pads would just run their own scope's cleanups then phi into those of the outer scope.

[marijnh]

They must not always jump to the cleanup of the outer scope — break and cont may cause some block cleanups to run, and then continue executing code.

[brson]

Related to #300

[pcwalton]

Also related (maybe obsoleted) by issue #2361.

[catamorphism]

@brson confirmed that #2361 supersedes this.