Function names aren't being exported on Windows

[rljacobson]

The function names weren't being exported, so I had to add __declspec(dllexport) and __cdecl to each function declaration in the extern "C" {} groups, in each class. Then I compiled under Visual Studio, and everything worked.

Originally posted by @ebaldino in https://github.com/rljacobson/Levenshtein/issues/8#issuecomment-837409719

I don't have access to this platform to implement this fix, but it should be pretty simple! I welcome pull requests. Also see issue #2.