Optimizes parsing of large non-UTF8 queries

Sources/GraphQL/Language/Source.swift

@@ -3,13 +3,19 @@
  * but is mostly useful for clients who store GraphQL documents in
  * source files; for example, if the GraphQL input is in a file Foo.graphql,
  * it might be useful for name to be "Foo.graphql".
+ *
+ * Note that since Source parsing is heavily UTF8 dependent, the body
+ * is converted into contiguous UTF8 bytes if necessary for optimal performance.
  */
 public struct Source {
     public let body: String
     public let name: String
 
     public init(body: String, name: String = "GraphQL") {
-        self.body = body
+        var utf8Body = body
+        utf8Body.makeContiguousUTF8()
+
+        self.body = utf8Body
         self.name = name
     }
 }