[String] Add Character.UTF16View and Character.UTF8View

Adds these collections, which are just String's views.

Tests added.

Author: milseman

stdlib/public/core/Character.swift

@@ -88,25 +88,26 @@ extension Character {
 }
 
 extension Character {
-  @usableFromInline
-  typealias UTF8View = String.UTF8View
+  /// A view of a character's contents as a collection of UTF-8 code units. See
+  /// String.UTF8View for more information
+  public typealias UTF8View = String.UTF8View
 
+  /// A UTF-8 encoding of `self`.
   @inlinable
-  internal var utf8: UTF8View {
-    return _str.utf8
-  }
-  @usableFromInline
-  typealias UTF16View = String.UTF16View
+  public var utf8: UTF8View { return _str.utf8 }
+
+  /// A view of a character's contents as a collection of UTF-16 code units. See
+  /// String.UTF16View for more information
+  public typealias UTF16View = String.UTF16View
 
+  /// A UTF-16 encoding of `self`.
   @inlinable
-  internal var utf16: UTF16View {
-    return _str.utf16
-  }
+  public var utf16: UTF16View { return _str.utf16 }
+
   public typealias UnicodeScalarView = String.UnicodeScalarView
+
   @inlinable
-  public var unicodeScalars: UnicodeScalarView {
-    return _str.unicodeScalars
-  }
+  public var unicodeScalars: UnicodeScalarView { return _str.unicodeScalars }
 }
 
 extension Character :

test/stdlib/Character.swift

@@ -347,6 +347,19 @@ CharacterTests.test("String.append(_: Character)") {
   }
 }
 
+CharacterTests.test("utf6/16/unicodescalar views") {
+  for c in testCharacters {
+    expectEqualSequence(String(c).unicodeScalars, c.unicodeScalars)
+    expectEqualSequence(String(c).utf8, c.utf8)
+    expectEqualSequence(String(c).utf16, c.utf16)
+
+    expectEqualSequence(
+      String(c).unicodeScalars.reversed(), c.unicodeScalars.reversed())
+    expectEqualSequence(String(c).utf8.reversed(), c.utf8.reversed())
+    expectEqualSequence(String(c).utf16.reversed(), c.utf16.reversed())
+  }
+}
+
 var UnicodeScalarTests = TestSuite("UnicodeScalar")
 
 UnicodeScalarTests.test("UInt8(ascii: UnicodeScalar)") {