For a 38-char UUID, require '{' and '}' characters

It was not intentional to simply ignore these characters, and examples/docs/tests all indicate that curly braces were the intended characters. See google#60

uuid.go

@@ -55,6 +55,9 @@ func Parse(s string) (UUID, error) {
 
 	// {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
 	case 36 + 2:
+		if s[0] != '{' || s[37] != '}' {
+			return uuid, fmt.Errorf("invalid prefix/suffix: got %q and %q, want '{' and '}'", s[0], s[37])
+		}
 		s = s[1:]
 
 	// xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx