Win32 support

[newhoggy]

Add support for Windows. See #129

[hazelweakly]

The store is not always in .cabal/store. Specifically, on windows, it's often been seen as a good idea to relocate the store to the root of the C drive to help prevent and work around long path issues (as well as APPDATA generally not being the best default for that). The haskell setup github action does this by setting the store path to C:\sr (a convention adapted previously by others, such as Stack).

This raises a few difficulties, because cabal doesn't have a sane way to query its configuration by calling the binary. The location of the store is given inside cabal's configuration file or the default. The default is .cabal/store but the cabal directory itself can have its location modified by an environment variable. The gory details are here

Cabal, the library, contains functions that will do the heavy lifting for you, but it's also quite heavyweight. Looking for the configuration file, reading it, and then determining the store from there (falling back to the default location otherwise) seems like the safest course of action if you don't want to use the cabal library.

A hacky one might be to check for the existence of cabal's store in a few hardcoded locations (eg ~/.cabal/store for linux and macos, and $env:AppData\cabal\store and C:\sr for windows.). Can't imagine the hacky solution being robust enough to be relied on, though.

[newhoggy]

dotCabalDirectory is actually missed named. It should actually just be cabalDirectory.

The value will be $HOME/.cabal on Posix and $APPDATA/cabal on Windows.

For cases where it is neither of these, the CI system can pass it in as a command line argument using --store-path.

[newhoggy]

I've verified this can sync to and from a local directory. I've not tested remote yet, but I don't see a reason why that shouldn't just work. If you like you can give this a spin and let me know if you encounter any issues.

[newhoggy]

Confirmed that remote sync on Windows works.