In order to help generate evolution files, mutations can provide hinted

versions of themselves. These are based on changes made to models that

aren't yet in the signature. To do this, each mutation class was

responsible for overriding __str__ and returning a string suitable for

the file, but this led to inconsistencies in how they serialized their

content.
This change standardizes all this. __str__ now calls

generate_hint(), which takes care of the base <MutationName>(...)

string and calls get_hint_params() to determine what parameters to

populate. That allows each mutation class to focus on the state instead

of the final construction of the string.

Unit tests pass.