swift234

[stdlib] Publicize Map, add lazy map()

As declared, Dictionary's keys and values were private.  Instead of
hiding Map away as _Map, give it a "nice" verbose name and expose it
through a "nice" lowercase global function called map(), which we
overload so it works on both Collections and Sequences, returning a
Collection when that's what it started on.

We'll follow this pattern for filter, which was requested on Array.  The
implementation is easy once you have a lazy view!

Swift SVN r18340