missing functionality for composition maps

[ThomasBreuer]

(motivated by #4471)

Currently maps of the type AbstractAlgebra.Generic.CompositeMap (and perhaps other types) do not support functions such as is_injective, is_surjective, kernel, and the unary version of image.

Eventually we want this functionality.

[fieker]

in order to get there we'd need to annotate more maps as homomorphisms, otherwise I'd say we can not do this...
Once we have homs, we can start to get there