Add convenience methods to mutator and other updates

[wschurman]

Why

This PR does a few things:

• Adds convenience methods to mutator to update enforcing so that a wrap with enforceAsyncResult isn't needed at the callsite. I've noticed this to be a common thing in usage.
• Adds setFieldWithBlock method to update mutator which allows setting a field with a function from current field value to new field value.
• Make ViewerContext not abstract. There's no reason this can't be concrete, and it'll help in tests.
• Disallow imports from index.ts since it just is cleaner IMO.

How

• Add features
• Add tests

Test Plan

Run tests

[ide]

Disallow imports from index.ts since it just is cleaner IMO.

Also mitigates cycles (which we can handle in most cases but generally nicer to avoid).