Track readers.
Track readers.

Every time a read is performed, track the calling MFA, so we are able to
know which completed functions should be re-executed if the state of the
lattice changes.

lasp-lang/lasp,lasp-lang/lasp,ergl/lasp,ergl/lasp,lasp-lang/lasp,lasp-lang/lasp,ergl/lasp,ergl/lasp