Various improvements made as they were needed:
* only gives the entity system table as argument in callback as that's the only thing needed most of the time
* to access the entity, a .entity field in now defined in every entity system table
* filter use ecs.any when given a table; allow booleans for always/never filter
* removed .m table from entity
* added ability to define methods on entities system table directly; allows to re-implement previous .m functionality (will provide some example systems in a later commit)
