Fix all Scala warnings in HKMC2

[Derppening]

Most warnings are mechanically addressable by either using alternative paradigms or removing redundant code.

The parts of this PR which require more scrutiny are:

• 90d09f5: Adding @unchecked to match arms, which may require more scrutiny since these will not be caught at runtime
• ab1ea34: Implement missing match arms by either (a) adding the appropriate logic (e.g. in showPlain), (b) adding a TODO(...), or (c) adding a lastWords or equivalent - Whether my additions are appropriate needs to be looked into.

Most difftests are unchanged - Some MatchErrors are changed into actual compilation errors.

[Derppening]

I think this PR should be ready for review. All warnings have been addressed.

[LPTK]

LGTM now, thanks!