WIP: Native client

[Quafadas]

The start of an effort, to get a mill Scala Native client.

Here's what (seems) to be in place.

1. Converted java -> scala
2. Removed api calls which are not native koscher
3. Proved nativeLink works
4. Tests are runnable

Unfortunately, I think this might be quite some way from a solution.

I've observed that the native client, appears to fire up a new server on every incarnation. Increasingly, I believe this may come down to some difference / bug / expected behaviour in how Scala Native interacts with files.

I can't seem to port some of the tests which handle file tailing operations, which seems to lend support to that idea.

If someone wants to play - on the branch of this PR branch.

mill runner.client.nativeLink

will emit a file at

out/runner/client/nativeLink.dest/out.

If I run an installLocalCache and run that file in the root of (say) a clone of upickle. I appear to get a (semi) functioning mill client. That starts a server every time (!)... but it does compile and run things. And it starts very quickly, so there;s that I guess.

for #4016

[Quafadas]

https://discord.com/channels/632150470000902164/635668881951686686/1312831702384640010