New Dump/Restore Format

[David Bremner]

Hi All;

There are some style/doc issues remaining, but because bugs in dump
and restore really suck, I thought I would ask for early feedback on
functionality.  I'm particularly interested in how the new dump format
works for weird message-ids (spaces and so on). If you have public
messages with tricky message-id's, I'd appreciate adding those
messages to the test suite.

Things to bikeshed now: name(s) of the formats; sup and notmuch are
maybe not ideal.  The format itself? The encoding format? The latter
is chosen for compatibility with nmbug, but we could discussing using
a bigger character set.

Things I know about
      
      - not enough tests
      - no man page, online docs.
      - no API docs for hex_encode/blah.

I think the code in hex-escape.[ch] is otherwise ready for (second)
review; I'll probably do another review of the code in
notmuch-(dump|restore).c myself for clarity, so you might want to wait
for the next round before diving in.

If you prefer pull from git, you can get these patches on branch "new-dump" 
at git://pivot.cs.unb.ca/notmuch.git