Queries and mutations are too similar

[Kashuab]

The following pairs of things are too similar:

• useQuery and useMutation,
• generated Query and Mutation classes
• QueryGenerator and MutationGenerator

I've always thought of queries and mutations as separate things, one is getting data, one is mutating data. Though perhaps in GraphQL they're practically the same? It'd be nice if we could find a nice way to unite them into one idea. mst-gql's React utilities combines the hooks into one useQuery, but it always felt weird to pass a mutation to it.

[Kashuab]

There are key differences, mainly when caching is involved. Removed MutationGenerator in favor of using QueryGenerator for both use-cases.