swift234

[stdlib] fix RangeReplaceable dispatch insanity

Before,

* We had a generic implementation of replaceSubrange that would
satisfy the protocol's last non-defaulted requirement

* A generic removeSubrange was in the protocol requirements needlessly

* The default implementation of that generic removeSubrange dispatched
  to replaceSubrange instead of the non-generic removeSubrange, robbing
  the model of optimization opportunities.

Now,

* There's one non-generic replaceSubrange that models must implement.

* The default implementation of removeSubrange uses replaceSubrange to
  do its work.

* Models can implement the non-generic removeSubrange if they can
  implement it faster than the default implementation.

* The generic removeSubrange uses the non-generic removeSubrange to do
  its work.