This replaces some derivation outputs by simple strings rather than
full Nix store paths. This removes the need to download the whole
derivation when all we need is a static string.
The [throw-keyids](https://www.gnupg.org/gph/en/manual/r2110.html)
option "hides the receiver of the encrypted data as a countermeasure
against traffic analysis." However, it also slows down decryption, and
even breaks some applications; see e.g.
https://github.com/open-keychain/open-keychain/issues/626
I think the sane default would be to leave it off, just as it is off
by default in gpg. The typical user will probably not need this level
of security, and will probably prefer a better user experience (faster
decryption and compatibility with a wider range of applications).
Closes#838
