The component methods system was awkward and didn't give much benefit compared to just using methods on Systems. Plus now we really only have data in entities.
Since we don't have component methods, the callback system had to be replaced; I integrated it with the default System methods since it's a relatively common behavior.
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)
