Simplify die so it's more JIT-able
WIP on Handler support.

This crashes after returning from the handler,
as I suppose it should, because I haven't
fixed hanlder invocation yet.
Support some kinds of handlers
Generalise throwish control.

Sometimes, the throwish op may throw to the same frame the source.
The simplest way of dealing with that by far is letting the
interpreter start the JIT code in the right place. But in that case
the invokish control guard will not do the right thing, so it has
to be replaced by two different guards.
Increase stack space by 32 bytes

This gives us some scratch stack space wherein
we can safely stuff temporary values even when
we're using stack arguments for C calls.
Merge branch 'jit-handlers' into moar-jit