Diff.evolution() is a fairly expensive operation that can get called

multiple times. It performs a diff of two signatures, generating a list

of mutations to transform one into the other.
This change caches the result of that so multiple calls won't require

the work to be repeated. After the initial generation, any future calls

are effectively free.

Unit tests pass.
Manually verified that repeated calls were getting the correct results

and not triggering a rebuild.