[String] Normalization-boundary-before UTF-8 fast path.

All latiny (<0x300) scalars have normalization boundaries before them,
so when asking if memory containing a scalar encoded in valid UTF-8
has a boundary before it, check if the leading byte definitely encodes
a scalar less than 0x300.

Author: milseman

stdlib/public/core/NormalizedCodeUnitIterator.swift

@@ -131,9 +131,11 @@ extension UnsafeBufferPointer where Element == UInt8 {
     if index == 0 || index == count {
       return true
     }
-
     assert(!_isContinuation(self[_unchecked: index]))
 
+    // Sub-300 latiny fast-path
+    if self[_unchecked: index] < 0xCC { return true }
+
     let cu = _decodeScalar(self, startingAt: index).0
     return cu._hasNormalizationBoundaryBefore
   }