Add support for `ValueCastable` formatting

[wanda-phi]

Currently (or, once RFC 50 is implemented), Values can be formatted and printed using the Format + Print mechanism. However, there is no support for pretty-printing custom ValueCastable such as data.View or Amaranth Enum. A mechanism for such should be designed and proposed via the RFC process.

[whitequark]

Should this be on 0.5 roadmap or not?

[wanda-phi]

Should this be on 0.5 roadmap or not?

shrug I don't really know. I think yes iff we can pull this off for lib.data + lib.enum without eg. getting blocked on support on the yosys side.

[whitequark]

OK, leaving off milestone for now.

[whitequark]

RFC for this feature: amaranth-lang/rfcs#58

[wanda-phi]

Obsoleted by #1243