Have less warnings when compiling the stdlib

[hamzaremmal]

This is the first step into removing all the warnings from the stdlib. It migrates the following:

• remove all [this] in private[this] and protected[this]
• All wildcards change from _ to ?
• Change infix cola of non-infix methods to the dot notation
• Add using keyword when needed
• Changes the location of classes to reflect their package hierarchy (61fb86e)
• Rewrite non-local returns (fcaa351)

[WojciechMazur]

LGTM

The change to private[this] / protected[this] fields might affect unsafe access to fields via reflection, but that's not an issue, since it's, well, private (AFAIR with private[this] we were accessing field directly, not using the accessor)

I haven't seen any cases of A with B <:< C, these are tricky since the correct rewrite is (A & B <:< C), but compiler rewrites might insert A & B <:< C which is interpreted as
A & (B <:< C). The one trickier case in ExecutionContext is correctly handled.