[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/String.swift

@@ -72,7 +72,7 @@ extension String : _BuiltinExtendedGraphemeClusterLiteralConvertible {
         UTF8.self,
         input: UnsafeBufferPointer(
             start: UnsafeMutablePointer<UTF8.CodeUnit>(start),
-            length: Int(byteSize)))
+            count: Int(byteSize)))
   }
 }
 
@@ -122,7 +122,7 @@ extension String : _BuiltinStringLiteralConvertible {
           UTF8.self,
           input: UnsafeBufferPointer(
               start: UnsafeMutablePointer<UTF8.CodeUnit>(start),
-              length: Int(byteSize)))
+              count: Int(byteSize)))
     }
   }
 }
@@ -320,7 +320,7 @@ extension String {
   ) {
     resultStorage.initialize(
         String._fromWellFormedCodeUnitSequence(UTF8.self,
-            input: UnsafeBufferPointer(start: start, length: utf8Count)))
+            input: UnsafeBufferPointer(start: start, count: utf8Count)))
   }
 }