Allow a test to claim to cover multiple constants/code-files

[nevinera]

Right now, we assume that a given ExampleGroup only provides coverage for a single code file (which defines a single testable constant). That's... usually ideal, but not always the case - classes get reopened, and sometimes a single test file is responsible for providing coverage for multiple classes.

We can support that situation by separating the conception of the coverage-delta from the spec-context, so that a given ExampleGroup (Context) can claim to cover multiple constants and/or paths, and then we can check the effective coverage provided for each of them. Effectively, we need to abstract the "claim" out into an explicit thing.