Rename `Eql` to `CanEqual`

[odersky]

Eql is a typeclass used in multi-versal equality to check that regular universal equality == is allowed. I believe therefore that CanEqual is a better name. That way, we do not raise the expectation that this is some sort of equality typeclass. And we avoid confusion and possible nameclashes if oneone we want to introduce an equality typeclass ourselves.

[sjrd]

This probably deserves a Contributors post, this late. Or at least tagging library maintainers who may have a skin in that game?

[odersky]

No code in the community build seems to have used this yet.