Add a type class for showing values at elaboration time

[krame505]

There isn't currently really a great way of printing out structured data at elaboration time for debugging purposes. I made a type class SShow mirroring CShow, except that it generates a String instead of Fmt for printing at elaboration time.

[quark17]

I had commented offline that it's unfortunate that this SShow library duplicates so much from CShow and wondered if one could be defined in terms of the other. I think I suggested that a function from Fmt to String could be used to define SShow, but I think you responded by pointing out that non-runtime types like List wouldn't have CShow instances. So then what about the other direction -- can we define CShow as just printing the string from SShow? Or can the choice of show versus $format be abstracted out, to have a common underlying polymorphic implementation? If not, this amount of redundancy isn't too bad, so I'm OK with accepting the PR as it is, if that's easiest.

FYI, I notice that there's no instance for the Array primitive -- is that possible to add?

[quark17]

Oh, also, could you add a test case?

[krame505]

There isn't a way to go from a runtime value to an elaboration-time String, either. If we had a typeclass StringLike like in Haskell, we could maybe abstract out the generic bit for showing data/structs/vectors, but I'm not sure how to define the primitive instances in that approach.

[krame505]

I added the Array instance and some test cases.