Restructure the "finder" functions.

Restructure the "finder" functions.

Previously, there were three different functions for the top entity,
initial state and test input. Now, there is just a single one, which
guarantees that these things are properly linked together. This should no
also support generating multiple entities at the same time (though there
is no top level interface for this yet).

This change also makes the testbench generation optional. A bunch of
functions were moved from Utils to GhcTools, to prevent a dependency loop.