[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

stdlib/core/ContiguousArrayBuffer.swift

@@ -153,7 +153,7 @@ public struct _ContiguousArrayBuffer<T> : _ArrayBufferType {
   func withUnsafeBufferPointer<R>(
     body: (UnsafeBufferPointer<Element>)->R
   ) -> R {
-    let ret = body(UnsafeBufferPointer(start: self.baseAddress, length: count))
+    let ret = body(UnsafeBufferPointer(start: self.baseAddress, count: count))
     _fixLifetime(self)
     return ret
   }
@@ -165,7 +165,7 @@ public struct _ContiguousArrayBuffer<T> : _ArrayBufferType {
     body: (UnsafeMutableBufferPointer<T>)->R
   ) -> R {
     let ret = body(
-      UnsafeMutableBufferPointer(start: baseAddress, length: count))
+      UnsafeMutableBufferPointer(start: baseAddress, count: count))
     _fixLifetime(self)
     return ret
   }