implement functions needed by C back end

[tromey]

The C back end assumes that a few special functions can be defined by Emacs to implement primitives like catch. See elcomp--c-direct-renames ... the names here are invented and can be changed, but what is important is implementing these in the C core.

• [x] :elcomp-fetch-condition: evaluates to the error value in the error branch of a condition-case
• [x] :save-excursion-save and :save-excursion-restore: save and restore excursion data. These might be very easy based on the implementation of Fsave_excursion
• [x] :save-restriction-save, :save-restriction-restore: Ditto for restrictions. Also maybe easy.
• [x] :unwind-protect-continue: Called at the end of an unwind-protect to re-throw the exception in flight
• [x] :catch-value: Return the value thrown via throw
• [x] PUSH_HANDLER is emitted in various spots but is gone from lisp.h
• [x] handler_matches: emitted by elcomp--c-emit-condition-case to handle case matching

[tromey]

Close but not quite done; still have some other bugs to fix in the generation.

[tromey]

condition-case is still wrong. it has to re-throw when no handler matches.

[tromey]

How handlerlist is popped is wrong, this has to be explicit in the IR