Add an extensible framework for adding built-ins

Add an extensible framework for adding built-ins

Now builtins can be written in assembly at the bottom, and they'll be
included and registered in the entry point on-demand by the program.
Only functions that are referenced will be included in the output.

This will need to be extended to handle functions which reference other
functions (like the handling of "true" and "false" if any of the
comparisons are included), but that's a matter of front-end changes.