Improve `FlxRandom` ASDoc

Plus some slight bug fixing (also removed two "untyped" warnings that I
missed). Pretty good turnout for a class I didn't test before
committing.
Deprecate old methods involving randomness

I didn't delete them, just moved them "out of the way" and added a
`@deprecated` flag, as well as a warning every time someone tries to
access them (see `FlxG.warnDeprecated()`).
Update existing references to deprecated methods to point to new members

Also did some performance tweaking, and in some cases, cut out large
chunks simply for the sake of clarity, nothing more.