An initial sketch of JITting extops calls.

Doesn't yet fully work, most probably because it JITs some that are
invokey without handling that, or that mess with interpreter state
(and so need a mechanism to flag them "don't JIT me").
check dll cache before searching through libpath
Some extra JIT-related flags for extops.

Put in master so we can add them to Rakudo's extops without having to
make a branch there.
Merge branch 'master' into jit-extops
Account for no_jit and invokish extop flags in JIT
Bring gethow JIT compilation in line with interp.

Now it needs a function call, as we may lazily deserialize the
meta-object.