Support function args being passed by reference

[joehoyle]

Currently it's not possible for a function to accept args by reference. This PR adds early support for doing so, by transforming any function args that are &mut T into args that are passed by reference. E.g.:

#[php_function]
fn my_function( arg: &mut Zval ) {
	arg.reference_mut().map( |zval| zval.set_bool(true) );
}