Explicit deprecation/removal notice for FHDLTestCase

[BracketMaster]

FHDLTestCase is no longer accessible in nMigen 0.3.
An attempt to import it should still fail, but let the user
know FHDLTestCase was intentionally removed.

Perhaps something to the effect of

Removal Notice: FHDLTestCase has been removed as per https://github.com/nmigen/nmigen/pull/399#issuecomment-638514027

Maybe Removal Notice could be highlighted in red?

[whitequark]

Given how many people turned out to (rather unexpectedly, for me at least) depend on it, I'm open to going through a full deprecation cycle for FHDLTestCase (i.e. it would be accessible in 0.3 with a warning, and removed in 0.4).

[awygle]

I think this is a good idea, and I also think it highlights that (given you clearly don't like FHDLTestCase, whitequark) we should prioritize designing its replacement (if any? not sure what your long-term plans are here). I know I'm using (a local fork of) FHDLTestCase in all of my nmigen designs, and most people I talk to are excited to use formal tooling with nmigen. This issue isn't the right place to track that of course so we should file a separate one if we don't have it already...

[whitequark]

given you clearly don't like FHDLTestCase,

It's not that I don't like it in particular. It's more that it was always intended as an internal API, and absolutely no thought went into its design. I assumed, wrongly, that people would know better than to dig out random stuff from the test/ subfolder...

we should prioritize designing its replacement

Sure.