Add support for Scala Native

[sluramod]

Cats (https://github.com/typelevel/cats) recently added support (typelevel/cats#1549) for cross-compiling to scala native.

Scala Native support in fs2 would be be very helpful.

[diesalbla]

This would of course depend on cats-effect support for Scala Native, as well. typelevel/cats-effect#1302

[armanbilge]

I started some preliminary investigation in armanbilge@20a1dbb based on typelevel/cats-effect#3057.

Crossing fs2-core is straightforward.

• porting java.util.zip.InflaterOutputStream to Scala Native would unblock compression

fs2-io is a lot more work. I focused only on the fs2.io.file package for now.

• java.nio.file.Path#register is missing
• so is java.util.stream.Stream#forEach

Without those, cross-compiling fs2.io.file will require complex splitting/platforming to remove Watcher (which is otherwise very nearly supported). We should port the missing APIs to Scala Native instead.

Furthermore, when running the fs2-io suite on Native I encountered the following failures.

X fs2.io.file.FilesSuite.permissions - innappropriate access should raise AccessDeniedException
X fs2.io.file.FilesSuite.delete - should fail on a non empty directory
X fs2.io.file.PathSuite.Monoid[Path]: monoid.right identity

The underlying bugs should all be fixed in Scala Native.

PRs:

• Path extends Watchable scala-native/scala-native#2749
• Port InflaterOutputStream from Android Luni scala-native/scala-native#2748
• Add j.u.s.Stream#forEach scala-native/scala-native#2747
• Drop empty elements in Paths.get scala-native/scala-native#2745
• Fix #2751: javalib Files#delete now throws informative DirectoryNotEmptyException scala-native/scala-native#2753

Issues:

• Write-opening a read-only file doesn't throw an exception scala-native/scala-native#2750
• Attempting to delete non-empty directory should throw DirectoryNotEmptyException scala-native/scala-native#2751

[armanbilge]

PR is up at #2975.