This necessitated making the JIT::Compiler aware of the current
Bytecode::Executable, since that's where all the string literals are
held, but that seems like a good thing.
These push a "valid" unwind context on the stack and check_exception()
now knows how to jump to the (catch) handler if present.
(finally) finalizers will require some more work, but with this change,
we now have basic support for try...catch. :^)
We now establish a stack of "unwind contexts" similar to what the
bytecode interpreter does, but here, it's a stack of structs with
addresses to the catch and finally blocks.
Unwind contexts also have a "valid" flag, and the root unwind context
(always present, pushed on JIT code entry) has valid=false, which we
interpret in check_exception() as "return and let our caller deal with
the exception".
Anything in Compiler that may generate an exception should now also
call check_exception() ASAP to emit the code for handling this.
