add ts_unify_def to compset in unifyLib
tweak compile_prog_def

make it return the reversed code (since the compiler generates bytecode
in reverse). I think this makes sense since we wouldn't expect to
add more to the code from compile_prog.
Build up a compset for evaluating prog_to_bytecode.

The various parts of this compset should be moved out into appropriate
libraries where they can be maintained and optimised. It is somewhat
laborious to put the compset together by hand, but the advantage is you
get complete control and inspection of what's in there, and can
experiment with turning evaluation of certain things on and off.