Use libsodium instead of OpenSSL for binary cache signing

Sodium's Ed25519 signatures are much shorter than OpenSSL's RSA
signatures. Public keys are also much shorter, so they're now
specified directly in the nix.conf option ‘binary-cache-public-keys’.

The new command ‘nix-store --generate-binary-cache-key’ generates and
prints a public and secret key.

perl/lib/Nix/Store.pm

@@ -17,6 +17,7 @@ our @EXPORT = qw(
     queryPathFromHashPart
     topoSortPaths computeFSClosure followLinksToStorePath exportPaths importPaths
     hashPath hashFile hashString
+    signString checkSignature
     addToStore makeFixedOutputPath
     derivationFromPath
 );