Make the Format class a Callable

So that this can work:

    use v6.e.PREVIEW;
    my constant &logger = Format.new("%04d - %s\n");
    for lines {
        m/.../;
        print logger $0, $1;
    }

src/core.e/Format.rakumod

@@ -3,7 +3,7 @@
 # contains a Callable that will be called with the CALL-ME method.
 # Otherwise acts as a normal string.
 
-my class Format is Str {
+my class Format is Str is Callable {
     has str @!directives;
     has     &!code;