This change tries to recover the performance regression in map() that
was caused by moving map() to a protocol extension and degrading the
static type information (when mapping a collection, we only know that it
is a sequence). Adding map() to the witness table allows us to provide
a specialized implementation for collections, and hopefully recover the
lost performance.
This is a speculative change, I don't have performance numbers. I will
watch the performance buildbots and if this change does not help, I'll
revert.
Swift SVN r27607
