mocking and testing infrastructure

[nikomatsakis]

#33 landed a very simple extension (set_unchecked) to support "fixing" the value of certain input queries for testing purposes. But as @matklad said in the comments of that PR:

I expect it would be useful to get some sort of the trace of the query, to make sure it does’t eval too much after invalidation, for example?

In general, one can imagine wanting to do fancier things than what set_unchecked permits, such as mocking up some inputs, test that you get the output you expect, and then alter the mocked inputs, and test (e.g.) that the output doesn't wind up being recomputed. This issue exists to record things we want and brainstorm on what that might look like.

[memoryruins]

update: set_unchecked methods and trait were removed in #140 and #141