Move generate_all_plausible_targets into common Holmake_tools.

Progress with #77
Move some 'evaluate logic in ML' files to new directory.

This code isn't being used and doesn't have any automatic test-cases,
making it a real candidate for removal.  However, for the moment, I'm
just shifting it into a directory of its own (rather than having it in
the unrelated src/datatype).