This allows getting rid of the ugly and error prone handling of
"bag of bytes" String in Remote.Helper.Encryptable.
Avoiding breakage like that dealt with by commit
9862d64bf9
And allows converting Utility.Gpg to use ByteString for IO, which is
a welcome change.
Tested the new git-annex interoperability with old, using all 3
encryption= types.
Sponsored-By: the NIH-funded NICEMAN (ReproNim TR&D3) project