Make it easier to cross call controllers by creating WP_REST_Requests #1218

Currently cross calling get_item etc is laborious and you need to know the route etc. Typically, a Request object does't need all that information, as the callback just wants to the arguments. For example, rather than internally creating a WP_REST_Request for /taxonomy/post_tag/terms/123, it would be nicer to just do new WP_REST_Request( array( 'taxonomy' => 'post_tag', 'id' => 123 ) ) or something.

@rmccue thoughts on this?