[stdlib] Have Substring.filter return a String (#10871)

* Have Substring.filter return a String

* fix spacing nits

new machine so editor misconfigured excuses excuses

* fix spacing nits

Author: airspeedswift

stdlib/public/core/Substring.swift.gyb

@@ -428,6 +428,12 @@ extension Substring {
   public func uppercased() -> String {
     return String(self).uppercased()
   }
+  
+  public func filter(
+    _ isIncluded: (Element) throws -> Bool
+  ) rethrows -> String {
+    return try String(self.lazy.filter(isIncluded))
+  }
 }
 
 extension Substring : TextOutputStream {

test/stdlib/subString.swift

@@ -92,6 +92,14 @@ SubstringTests.test("Comparison") {
 		["apple", "pen", "pen", "pineapple"])
 }
 
+SubstringTests.test("Filter") {
+  var name = "😂Edward Woodward".dropFirst()
+  var filtered = name.filter { $0 != "d" }
+  expectType(Substring.self, &name)
+  expectType(String.self, &filtered)
+  expectEqual("Ewar Woowar", filtered)
+}
+
 SubstringTests.test("CharacterView") {
   let s = "abcdefg"
   var t = s.characters.dropFirst(2)