[io grant] Allow no-arg &prompt

It's not entirely unheard of to not want to include any text message
to the user when prompting for some input. It's a bit antisocial to
force the user to specify `''` arg for such cases.

Worse, they might introduce a subtle bug into their code by switching
to use `get` for a message-less `prompt`, but `get` defaults to
`$*ARGFILES`, instead of `$*IN`

src/core/io_operators.pm

@@ -80,7 +80,10 @@ sub gist(|) {
         !! nqp::p6bindattrinvres(nqp::create(List), List, '$!reified', args).gist
 }
 
-sub prompt($msg) {
+multi sub prompt() {
+    $*IN.get
+}
+multi sub prompt($msg) {
     my $out := $*OUT;
     $out.print($msg);
     $out.flush();