Link chk.hs into the Android build

https://gitlab.com/exarkun/chk.hs is the Haskell library for doing Tahoe-LAFS CHK. It currently implements only encoding - which is not the piece we need right away, but the decoder will be there at some point as well.

We need to be able to depend on this library from PrivateStorageMobile. This might be trivial ("add it to the .cabal file, done") or we might have to fight with some dependency versions ... or maybe something else, even, I dunno.

Even before we're using it for anything real, we can try to get this dependency building in the PrivateStorageMobile codebase. Then we'll be ready to use the decoding functionality when it shows up.

This might involve changes to either or both of PrivateStorageMobile and chk.hs.