Materialization conflict

[lydia-duncan]

When two modules that are used in a single program try to access
two separate external functions that share a name, the first function
accessed is used in both cases. I think that the materialization of
functions (and use of cached materializations) should take into
account the source location of the external function, so that
if the current source and the previous source differ, both will be
accessed under the correct circumstances.

See #30, test_materialization_conflict_[A|B].py