[RTG][Elaboration] Add support for 'index.add' and 'index.cmp'

[maerhart]

• Use the index dialect instead of arith because it is a smaller dependency and simpler, but still has everything we need.
• Add support for index.add and index.cmp
• Add specialized elaborator values for indices and booleans because creating attributes for intermediates is quite bad performance-wise (ideally we'd have some evaluation interface at some point that we can also use for folding, or better folders that we can call directly)

[darthscsi]

This seems like just a std::variant.

[maerhart]

I'll address this in #7997

[darthscsi]

LGTM. I think the elaboration is reinventing std::variant, but that is separate from this PR.