Support for .execute method at Connection level

Prepared statements can be expensive or unavailable for some
drivers, so Connection.do can skip 'prepare' when no @params given
and the driver support direct execution implementing the .execute
method in its Connection class.
MySQL in particular can't prepare some kinds of statements.

lib/DBDish/Connection.pm6

@@ -45,7 +45,9 @@ method new(*%args) {
 method prepare(Str $statement, *%args) { ... }
 
 method do(Str $statement, *@params, *%args) {
-    with self.prepare($statement) {
+    if !@params && self.can('execute') {
+	self.execute($statement, |%args);
+    } orwith self.prepare($statement, |%args) {
 	LEAVE { .finish }
 	.execute(@params, |%args);
     }