[stdlib] Grapheme break fast-paths for Cyrillic, Arabic, Hangul

Add in more grapheme break fast paths for scripts based on Cyrillic,
Arabic, or Hangul. Generates significant performance wins, similar to
those for the unihan fast paths.

While every extra check does slow down the runtime of
_internalExtraCheckGraphemeBreakBetween as currently implemented, I've
not found the performance cost to be relevant for workloads with
occasional mixed emoji contents, nor for workloads that his the
earlier checks. A pure Korean workload (currently the last check) does
pays a rather noticable price for the previous checks, but this is
only because the workload is now so greatly improved. Optimizing this
implementation is interesting future work, but not urgent.

Author: milseman

stdlib/public/core/StringCharacterView.swift

@@ -301,29 +301,47 @@ extension String.CharacterView : BidirectionalCollection {
     // satisfying this property, has a grapheme break between it and the other
     // scalar.
     func hasBreakWhenPaired(_ x: UInt16) -> Bool {
-      // TODO: This doesn't generate optimal code, tune/re-write at a lower level.
-
+      // TODO: This doesn't generate optimal code, tune/re-write at a lower
+      // level.
+      //
+      // NOTE: Order of case ranges affects codegen, and thus performance. All
+      // things being equal, keep existing order below.
+      switch x {
       // Unified CJK Han ideographs, common and some supplemental, amongst
       // others:
       //   0x3400-0xA4CF
-      if 0x3400 <= x && x <= 0xa4cf {
-        return true
-      }
+      case 0x3400...0xa4cf: return true
+      // TODO: CJK punctuation
+
+      // Repeat sub-300 check, this is beneficial for common cases of Latin
+      // characters embedded within non-Latin script (e.g. newlines, spaces,
+      // proper nouns and/or jargon, punctuation).
+      case 0x0000...0x02ff:
+        // Conservatively exclude CR, though this might not be necessary from
+        // previous checks.
+        return x != _CR
+      // TODO: general punctuation
 
-      //
       // Non-combining kana:
       //   0x3041-0x3096
       //   0x30A1-0x30FA
-      //
-      // TODO: may be faster to verify whether only 3099 and 309A don't have
-      // this property, and compare not-equal rather than using two ranges.
-      if 0x3041 <= x && x <= 0x3096 || 0x30a1 <= x && x <= 0x30fa {
-        return true
-      }
+      case 0x3041...0x3096: return true
+      case 0x30a1...0x30fa: return true
+
+      // Non-combining modern (and some archaic) Cyrillic:
+      //   0x0400-0x0482 (first half of Cyrillic block)
+      case 0x0400...0x0482: return true
+
+      // Modern Arabic, excluding extenders and prependers:
+      //   0x061D-0x064A
+      case 0x061d...0x064a: return true
 
-      // TODO: sub-300 check would also be valuable, e.g. when breaking at the
-      // boundary between English embedded in Chinese.
-      return false
+      // Precomposed Hangul syllables:
+      //   0xAC00–0xD7AF
+      case 0xac00...0xd7af: return true
+
+      default: return false
+      }
     }
     return hasBreakWhenPaired(lhs) && hasBreakWhenPaired(rhs)
   }