'SimpleUnitId' constructor renamed to 'UnitId', and augmented
with a new field 'Maybe String' recording a hash that uniquely
identifies an instantiated unit of the library 'ComponentId'.
'UnitId' can't be used to represent partially instantiated
unit identifiers; see Distribution.Backpack for how we handle that.

Previous uses of 'SimpleUnitId' should now use 'newSimpleUnitId'.
'unitIdComponentId' folded into a record selector for 'ComponentId'.