WIP: split core functionality into UnitfulBase

[Gregstrq]

This PR adresses issue #424 and issue #359.

• I have moved all the files except "pkgdefaults.jl" and "dates.jl" into a subdirectory package UnitfulBase.

• I also moved the definitions of radian, steradian and degrees into UnitfulBase. The reason for that was the extension of the function mod2pi which uses degrees. I have decided to move degrees into UnitfulBase instead of keeping mod2pi extension in Unitful. Since radian and steradian are dimensionless and thematically connected to degrees, I have moved them as well.

• Unitful is now a registered extension of UnitfulBase.

• I have edited the tests so they work with the splitted UnitfulBase. I have run the tests on my computer: they run successfully with the exception of the single test for issue @u_str macro hygiene? #272 (see these lines). The problem is that if @u_str is imported without at least importing Unitful, Unitful is not registered as an extension of UnitfulBase.

• Since UnitfulBase is not a registered package yet, the only way to add it to Unitful project environment was to dev it. The information about the path to UnitfulBase was stored only in Manifest.toml, which I did not commit. As a result, the tests might be broken in CI for now. If this PR will ever get merged, the problem will disappear once UnitfulBase is registered as a package.

I did not touch the docs yet. I would like to hear your opinion about the contents of this PR first.

[Gregstrq]

I understand that the idea of the splitting of the core functionality is an arguable thing.

I created this PR as an experiment to poke around and to see what we get.
That said, I am eager to work on any feedback from the maintainers.
(May be, a good approach is to create an experimental branch and push this PR there instead of master.)