Crate milter_callback

Procedural macros to generate C callback functions and a thin wrapping layer.

Work in progress.

Attribute Macros

on_abort
on_body
on_close
on_connect
on_data	Generates a C callback function wrapping this handler function.
on_eoh
on_eom
on_header
on_helo
on_mail
on_negotiate
on_rcpt
on_unknown