Replace Mock.InnerMocks with InnerMockSetup

[stakx]

Mock.InnerMocks is the mechanism that mocks use to keep track of subordinate ("inner") mocks for recursive verification. This data structure causes quite a few problems:

1. Having both setups and the InnerMocks dictionary for mock members is difficult to understand, especially as the two structures are not necessarily kept in perfect sync.

2. One example of this is when a setup for some mock member has been overridden, but there is still an InnerMocks entry for that same member. This typically leads to "unreachable" inner mocks being included in recursive verification when they should no longer be included.

3. InnerMocks is a simple dictionary keyed by method. Unlike setups, it does not store argument information and is therefore much less precise. There can only be one registered inner mock per method.

We can keep track of inner mocks as a special type of setup to solve all of these problems.

This PR takes care of (1) and creates the basis for (2) and (3).