Cross-build kernel+std for Scala Native

[armanbilge]

As proposed in #1302 (comment). Note: there is currently no laws or testkit support, this will require coop and MTL to join us on Native (probably doable). Also there is no specs2 for Native, probably the best fix here is for CE to migrate to munit :(

[djspiewak]

I think my main concern with this is the fact that Scala Native still doesn't support Scala 3, which in turn means that all of the build machinery becomes very… bespoke. For example, we can no longer run +test, or +ci. We see some of the annoyances with this on the Cats build, which are ameliorated by the validate aliases, but it still represents an ongoing maintenance pain.

I'm certainly open to being convinced, but I have a hard time believing that this maintenance pain is justified given the low current rate of adoption of Scala Native.

[armanbilge]

I agree and think we should wait to cross at least until it's a little less painful (ahem Scala 3 :) and/or there's a runtime available. There's just enough here that anyone who wants to play around with implementing a native runtime can fork my branch, publishLocal, and give it a go.