integrates Paul's scala-hash into my toolchain

`scala-hash <committish>` now downloads Scala artifacts corresponding
to the specified commit and prints out Scala home for the artifacts.

`scala|scalac|... -hash <committish>` now runs `scala-hash` and then
uses its result to launch the corresponding tool. Obviously, -hash
overrides scala-root, but other things like scala-toolcp and scala-options
can still be customized.

`sbt -scala-hash <committish> -scala-version <corresponding version>`
now runs sbt with the build of Scala that corresponds to the provided version.