[5.6] Signed routes with routable parameters

[mattdfloyd]

This adds the ability to pass in a parameter that implements the UrlRoutable interface when generating a signed route.

Given

class Invoice extends Model
{
    ...
}

and

Route::get('invoices/{invoice}', 'InvoiceController@show')->name('invoices.show');

then

return url()->signedRoute('invoices.show', Invoice::first());

will generate a signed url for the invoice.