Add -e option to sagai-exporter to set output encoding, truncate & pad
comment field properly (a followup to 24c49eb04)
Properly pad COMM3 when it's empty
Fix COMM3 padding
Field combinators now push bytestring to ExportMonad state.

This allows us to recode bytestrings to target encoding as they are
added to SAGAI entry contents. `push` is the new `return` for field
combinators. `pushRaw` is provided for use with comment fields.
Drop unused error type
Pad comment fields using space character in output encoding, not UTF-8
Merge from sagai-monadic-push branch