Fast return handling draft
FIXME note
Test case trivia (semicolons)
Disable fast return paths for 1.0 release

This feature needs some more thought and has somewhat minimal performance
impact at present (10-15%) over the longjmp path.  The better approach may
be to handle every return as a "fast return" and fall back to a longjmp in
special cases like thread yields and such.
Incomplete test case for fast return code paths
Merge branch 'disable-fast-return'

Disable fast returns for 1.0 release: they have quite a small speed impact
(10-15%) at present and return handling (and other call handling code) needs
a review anyway.