calling a module homomorphism is type-unstable

MWE:
```julia
julia> using AbstractAlgebra, Test

julia> M = FreeModule(ZZ, 1);

julia> f = ModuleHomomorphism(M, M, identity_matrix(ZZ, 1));

julia> m = only(gens(M));

julia> @inferred f(m)
ERROR: return type AbstractAlgebra.Generic.FreeModuleElem{BigInt} does not match inferred return type Any
```