Check other fields are consistent in `LoanPath::common`.
Added type fragmentation code to compute drop obligations remaining post move.

Add differentiation between assigned_fragments and moved_fragments.

Computes the fragment siblings, prep to add them to dataflow inputs.

Added 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.
Added doc for the fragments work as it stands today.