improve compset theorem for DeclAssum

It might not be necessary to evaluate DeclAssum at all, but this change
shouldn't hurt either way. What we want to avoid is attempting to
evaluate prim_sem_env by appealing to its definition (but since it came
from Lem it's hard to zDefine it directly).
add some debugging output to the translator

the debugging definition of print_time is currently commented out, but
uncomment it to get more information about what the translator is doing
and how long it is taking.
performance hacking snoc_cenv_eq_thm Dtype case

The slow part previously was the evaluation of
  ALL_DISTINCT (MAP FST (build_tdefs ... ++ ...))

The change is to use a custom compset instead of EVAL, where this
compset only has lists, strings, pairs, and build_tdefs.

The key, however, seemed to be to remove LIST_TO_SET_THM from the list
compset, because it ruins the nice rewrite for MEM. This should probably
be considered upstream.

This commit also includes some commented out debugging stuff, and a
commented out previous attempt to speed things up by caching
ALL_DISTINCT theorems.