minimail

Note on symbol names:
- Private symbols start with the "empty prefix" consisting of a
  single dash.  This is expanded via the shorthand mechanism to the
  usual `minimail--' prefix.
- Public symbols are always written in full and have the
  `minimail-' prefix.
- The `athunk-' prefix is expanded to `minimail--athunk-'.  This
  part of the code is independent and could be moved to a separate
  package.