[Refactor] Introduce TranslatorContext (2/2)

[czechboy0]

Motivation

Depends on #638, so don't review until that lands.

Continuation of #638, broke up into multiple PRs for easier review.

Also a non-functional change, purely a refactoring.

Modifications

Changed a bunch of TypeMatcher methods from static to being instance methods, which gives them access to the translator context.

Moved a few other methods that depended on the static methods to be extensions on the closest type that already has access to the context.

Result

Now TypeAssigner/TypeMatcher can take runtime configuration, e.g. through feature flags.

Test Plan

All tests still pass.

[simonjbeaumont]

I'll wait until the other PR lands, but will approve it when it's all rebased.

[czechboy0]

I'll wait until the other PR lands, but will approve it when it's all rebased.

Okay, ready to go.