swift234

[stdlib] Protocol Extension-ize CollectionType

Eliminates _CollectionDefaultsType and _CollectionGeneratorDefaultsType
and creates sanity that should allow us to Protocol Extension-ize the
lazy operations, which have been blocked on inscrutable errors due in
part to the existing complexity.

Note: names beginning with "_prext_" are intended to be public after API
review and removal of the "_prext_" prefix.

Swift SVN r29689