support analyzing diffs for framework or assembly changes

Would be nice to generalize what we have for jit diffs to handle cases where the framework (or some assembly) has changes. This is somewhat doable today, but clunky:
```
jit-diff diff --pmi --diff --assembly a/x.dll -t a
jit-diff diff --pmi --diff --assembly b/x.dll -t b
jit-analyze --base bin/diffs/a/diff --diff bin/diffs/b/diff
```
Though for corelib it is more involved as we currently can't load a different copy of corelib (perhaps solvable with assembly load contexts?).

Analysis may need tweaking since we will see more mismatches in methods. And we might want to publish overall per-assembly summary data.

Would also be nice if via reflection we could enumerate all instantiations of generics, so that the analysis also highlight refactoring that increases or reduces the net amount of instantiation needed. My understanding is that this is not possible today.