Make a few changes to function and parameter names

[dturner]

I have:

• Moved the core API types (e.g. Mutation) into a file named Mutator.kt.
• Renamed the builder function mutation to mutationOf to disambiguate it from the Mutation type, and because it's more consistent with other builder functions like flowOf, listOf and mutableStateOf.
• In the Producer types and associated builder functions, renamed the parameter mutableFlows to inputs to better indicate that the purpose of this parameter is to provide the state inputs which are then mutated/transformed into new states.