Add NarrowTypes transform that converts columns to the most specific types possible

[juliohm]

Add a transform that takes a table with possibly generic column eltypes and returns a new table with specific column types.

[eliascarv]

The PR #96 depends on fixing the following issue from the Transducers.jl package:
JuliaFolds/Transducers.jl#524
Implementation will be delayed until this issue is fixed.

[juliohm]

I think we don't need this transform. We can assume that the user will load a table with appropriate narrow types, and that our pipelines will ensure type propagation. If something is not working as expected due to some compiler or language limitation, we can refactor the code to achieve the desired results.