Optimize some commonly used utf8 functions by:

- using counted loops instead of searching for terminating null. In
  the important cases the caller already knows the length: change
  the API to pass it in. Keep the old API version as well to avoid
  extensive changes to non-critical debug and test code.

- ensure the common cases are at the start of if/then/else chains.
  Usually 99+% of characters are ASCII even in mixed strings.

- for the "convert" functions, when both utf8 and utf16 lengths are
  passed, and are equal, it means the entire string is ASCII, and a
  specialized loop can be used. The compiler might then unroll or
  even vectorize this.

The functions improved are (tested on Nexus 5 with a 44 character
ASCII string):

CountModifiedUtf8Chars    :  20% faster
ConvertUtf16ToModifiedUtf8:  80% faster
ConvertModifiedUtf8ToUtf16: 200% faster

Also for completeness CountUtf8Bytes has been cleaned up a little, but
the speed is unchanged. Unlike CountModifiedUtf8Chars, it was already
passed the length, rather than searching for null.

Change-Id: I1c9b7dea3eda869fc9f5f6b4dd6be8cdd5bc3ac0