Restore the deleted ML, Ocaml and MLton target directories -- these are
needed because EmitML fails when target directories don't already exist.
Also added sum_emitTheory and option_emitTheory to emitLib.

Have also moved the EmitTeX stuff into a separate sub-directory. Added
this to the build sequence.

The function datatype_thm_to_string has been moved from EmitTeX to emitLib.