Check other fields are consistent in `LoanPath::common`.
Added fragments.rs: compute drop obligations remaining post moves.

Includes differentiation between assigned_fragments and
moved_fragments, support for all-but-one array fragments, and
instrumentation to print out the moved/assigned/unmmoved/parents for
each function, factored out into separate submodule.
First tests making use of the new fn move-fragments instrumentation.

The tests use new "//~| ERROR" follow syntax.

Includes a test for moves involving array elements.  It was easier
than i realized to get something naive off the ground here.