Introduce compose, remove extend

[mweststrate]

Replaces extend, which is to be removed.

Proposed api:
const NewModel = types.compose(BaseModel, newProps, newActions)

1. Note that this does not establish an inheritance relationship; it just takes the defiinition on BaseModels and constructs a new model form it.
2. reconfiguring props is not allowed
3. reconfiguring actions is allowed.
4. "Supercalls" can be made by using BaseModel.actions.actionName.call(this, args)
5. baseModel.isAssignableFrom(NewModel) will yield false. If you need a place where both types are allowed, use union(NewModel, BaseModel)

[RainerAtSpirit]

How would computeds be handled in with this proposed API?

[mweststrate]

Good question. I think newProps should allopw overriding computeds from BaseModel

[mweststrate]

reusing computed from "super" if overriding will probably be tricky though, that requires a helper function

[mweststrate]

Released as 0.8.0