WIP: Loom support

[jdegoes]

Add experimental support for Loom, including:

1. Using the default Loom scheduler
2. Backing every fiber with a virtual thread (guaranteed 1-to-1 mapping)
3. Embracing blocking wherever relevant, e.g. when converting a Java Future to a ZIO effect
4. Eliminating or minimizing synchronized in favor of non-OS-thread blocking constructs like Lock
5. Introducing dual code paths for every ZIO.async, such that async is no longer used under Loom
6. Refining ZIO semantics in cases where that's necessary (e.g. green threads cannot shift!)

[guersam]

Wouldn't it make the effect run multiple times due to the re-evaluation of the side effecting named parameter => Future[A]?

[WtzLAS]

any further progress on this?

[domdorn]

as just mentioned in the spartan session, maybe this
https://index.scala-lang.org/sbt/sbt-multi-release-jar
could allow you to ship one jar with different behavior for loom-aware and non loom-aware JVMs.
E.g. I'm just upgrading all our services to JDK21, so we could already use this there.

[devcdcc]

Hello @jdegoes @adamgfraser any update about it?
:c