Fix bugs in async exception raising: instead of trying to build an
application of the exception handler to the exception directly, just
leave a THUNK(raise,exception) on top of the CATCH_FRAME ready to
trigger the next time this thread is run.

This avoids two problems: the PAP we were using before wasn't really a
PAP, which broke some assumptions elsewhere (c.f. PAP_ENTRY:
CATCH_FRAME failure), and there was also some duplication between
raiseAsync and raisezh_fast due to the fact that we were attempting to
do the raising directly.