[stdlib] Speed up char iteration on ASCII strings.
[stdlib] Speed up char iteration on ASCII strings.

By doing a fast check for CR-LF, we can speed up forwards and
backwards character iteration on ASCII strings by ~2-3x. This speedup
can be seen in the stringTests suite (previous PR).

It is done as a fast path inside of
_measureExtendedGraphemeClusterForward/Backward, which is never
inlined. Doing the fast path inline would yield even more dramatic
improvements, and might be an area for future exploration, but would
slightly pessimize non-ASCII strings due to code bloat. Additionally,
the current implementation has not been micro-optimized yet. It's
possible that more optimal code would be smaller and have less of an
impact on the general case.

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