swift234

[stdlib] Prototype new protocols for Sequences, Streams, and Containers

These protocols fix a number of problems with the existing system,
among them esoteric naming ("Indexable", "Generator"), the subtlety of
a MultiPassGenerator protocol that refines Generator without adding
any structural requirements, and questions about whether types with
reference semantics even fit into the framework.  Lacks any real
testing; this merely proves that we can compile the protocols and use
them in trivial algorithms.

Swift SVN r10967