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