Fix encoder output types

[EssamWisam]

It turns out that most encoder or transformers implemented in here were subject to the bug mentioned in FluxML/MLJFlux.jl#275 . This PR attempts to fix the output types for all the encoder/transformer methods implemented in this package.

In doing so, adherence to the following performance tip in the Julia doc was considered:

If you cannot avoid containers with abstract value types, it is sometimes better to parametrize with Any to avoid runtime type checking. E.g. IdDict{Any, Any} performs better than IdDict{Type, Vector}

which can be observed in the target, frequency and ordinal encoder changes. It could be helpful to consider changes commit by commit.

[EssamWisam]

Merging this should close #20

[ablaom]

Thanks for catching this and adding all the new tests.

[ablaom]

Looks good to go, thanks again.