new bytecode emitter, GenBCode (2nd attempt)


Compared to the previous submission, this PR adds functionality to GenBCode in stages, with additional documentation.

Testing against the community build will provide more information than isolated tests.
Bytecode tests are by their very nature fragile (e.g., testing first that pattern matching emits a chain of jump, only to make sure a moment later that chain of jumps has been collapsed by GenBCode).

However concrete ideas and examples on how to balance usefulness vs brittleness are welcome.

And of course, reviews about the content proper of the PR are most welcome.