we'll also run the tests after which is great, but this at least gets us some
automatic checking of our executables which aren't really covered by automated
tests

Specifically, this will build our new `make-keypairs` executable.

Take tahoe-chk as a dependency because it can already represent some of the
fields of an SDMF share.  Also, a crypto library for key representations.

Fix a typo in the cabal file that passed with only a warning before.