Refactor platform detection internals

[mfelsche]

With merging of #15 we now have a mixture of ifdef and if (in case of osx) for detecting how to generate application directories.

It might be cleaner to use a primitive like:

primitive Osx
primitive OsxAsUnix
primitive Unix
primitive Windows

type Platform is (Osx | OsxAsUnix | Unix | Windows)

and use a match statement in each function to determine how to create the particular appdirs.

cc @SeanTAllen does that express your ideas?

[SeanTAllen]

@mfelsche yes.

a key point being that we determine the platform once on startup and then use that platform type with matching everywhere else.