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.

[smarter]

The only remaining item is the usage of sun.misc.Unsafe in LazyVals, but I had a look and trying to abtract over this to use VarHandle on Java 9+ instead looks very complicated and will involve some important changes to the lazy val implementation, this is likely to be a wasted effort since we're anyway going to completely change it post-3.0 with #7140, so I'll be closing this and we'll have to handle this at the same time we deal with #7140.