util/crypto: _notmuch_message_crypto: tracks message-wide crypto state

E-mail encryption and signatures reported by notmuch are at the MIME
part level.  This makes sense in the dirty details, but for users we
need to have a per-message conception of the cryptographic state of
the e-mail.  (see
https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html for more
discussion of why this is important).

The object created in this patch is a useful for tracking the
cryptographic state of the underlying message as a whole, based on a
depth-first search of the message's MIME structure.

This object stores a signature list of the message, but we don't
handle it yet.  Further patches in this series will make use of the
signature list.

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