Make all arg-like APIs take os.Shellable instead of Seq[String]

[lihaoyi]

They mean the same thing, but os.Shellable is much more ergonomic: you can pass in os.Paths, numbers, Seq[String]s, Seq[os.Shellable]s, Options, PathRefs and it all just works.

Might need to do a bit of digging to figure out what APIs need to be updated, but all the runSubprocess APIs are candidates, mainArgs/jvmArgs, etc. This should let us clean up tons of boilerplate .path.toStrings everywhere

Might need to wait till 0.13.0 when we can break bincompat, but may be able to do it earlier with some careful overloading