The check that "build_composite_env composites = OK (make_composite_env composites)" is taking time exponential on the number of struct/union definitions, at least on the example provided in #196.

The solution implemented in this commit is to use computational reflection more efficiently, just to check that "build_composite_env composites" is of the form "OK _".  From there, a new function Clightdefs.mkprogram produces the appropriate Clight.program without additional computation.