FlxRandom: allow creating instances

Per @joonjoonjoon's request, this allows developers to create an instance of `FlxRandom` with its own seed and determinism. Basically `FlxRandom` now uses a singular `FlxRandomInstance` internally, and all of the `FlxRandom` logic moved into `FlxRandomInstance`.
Right. Those functions aren't really part of the public API / relevant to the user though.