Clean-up scala.runtime

[anatoliykmetyuk]

As per the stdlib review discussion we had a few weeks ago, here's what needs to be done with scala.runtime package:

• Rename Tuple.scala -> Tuples.scala
• Move everything from dotty.runtime to.runtime – everything move
• LazyVals: don't rely on Unsafe in Java 9 – copypaste Scala 2 stdlib code & detect usages
• Remove TupledFunction
• Function$ classes – investigate

[bishabosha]

should scala.quoted.runtime become scala.runtime.quoted?

[smarter]

I think either is fine.