swift234

Replace stray "Enumerable"s with "Sequence"

There were a lot of instances of "Enumerable" that really should have
been changed already.  Along the way, I also

* Moved the adaptor that allows a Stream to be used as a Sequence into
  CompilerProtocols.swift; it's just an affordance to work around
  temporary inability to express the Stream protocol properly

* Made a trivial simplification in Algorithm.swift

Swift SVN r11946