[prove] keep further hints on each saving

During intermediate saves, previous hints for not-yet proved goals are lost, until the final dump. Hence, when hitting Ctrl-C, previous hints are lost.

The buffer (dumper) is first initialized with all proof hints. Then, each theory filters its results to keep only its goals. Finally, each goal result is overwritten.

Also puts a minimal 100ms delay between consecutive saves (but the last one).

And finally add a (very simple) support for goal selection : -g Module or -g Module.goal (can be repeated).

Fixes #12 (closed)