Implements a new block return optimization. It targets frame
unwinding in the cases of returning from (nested) blocks like these:
We were throwing an exception in such cases to unwind the block frames.
This is unnecessary because we already handle other kinds of
control-flow (retry) in call sites with a block (x.times and y.times in
the example above). So adding one more check (for frame unwinding
return) there enables us to not throw an exception and doesn’t slow down
the fast path (no cross-frame control flow).
Removes an unnecessary allocation in dynamic call sites that
dispatch to a Ruby method and take a block.
Adds targeted unit tests to increase code coverage of control