dillon1000/react

Rename Effect.Mutate -> ConditionallyMutate

We currently use `Effect.Mutate` both for places that _may_ mutate (ie untyped 
function calls) and for places that have known mutation (typed function calls, 
or operations like `delete x.y`). We then use a separate mechanism to decide 
whether to reject the input, with some call paths checking the effect and others 
not. 

This stack refactors this logic in InferReferenceEffects per our discussion, so 
that `Effect.ConditionallyMutate` is for "may or may not mutate" either because 
we're not 100% sure (untyped function) or because the mutation depends on the 
operand (ie, a callback arg that will be invoked and thus will mutate if the 
lambda is mutable, not mutate if the lambda is immutable). Later diffs add back 
`Effect.Mutate` as "definitely 100% mutating".