swift234

[stdlib] UnsafeBufferPointer: add ".count"

In answering a forum post I noiced that I wanted this and it was
missing.

Also, extensive comments

Also, rename the length: init parameter to count:.  When writing the
comments for the init function it became painfully clear why we use
"count" is better than "length" especially around pointers and memory:
the former is much less easy to mistake for "length in bytes".  Plus
it's consistent with the new ".count" property

Swift SVN r20609