Feature Request: Common metric class traits

[dgkf]

Originally posted by @dgkf in #43 (comment)

This idea boils down to having combinators that add new validators to an S7 class and apply additional constraints, so that

class_integer + scalar + finite

Actually produces a new class object with additional validators.

It's quite exploratory, but it seems like there's interest in something like this. There's some talk in S7 about adding scalar classes -- so this might be supported out-of-the-box in the future (RConsortium/S7#554).