swift234

stdlib: make all collections sliceable, first steps

This commit adds a minimally working Slice type and minimal tests.  Even
though it might seem logical how the new protocol requirements are
injected in the CollectionType hierarchy, it is very fragile and
required many attempts to get the typechecker to finally accept it.
Because I want to ensure that the type checker does not regress, I'm
commiting a patch that does not do much yet.

Swift SVN r27617