Introduce a generic tree-like abstraction for MIME traversal.

Introduce a generic tree-like abstraction for MIME traversal.

This wraps all of the complex MIME part handling in a single, simple
function that gets part N from *any* MIME object, so traversing a MIME
part tree becomes a two-line for loop.  Furthermore, the MIME node
structure provides easy access to envelopes for message parts as well
as cryptographic information.

This code is directly derived from the current show_message_body code
(much of it is identical), but the control relation is inverted:
instead of show_message_body controlling the traversal of the MIME
structure and invoking callbacks, the caller controls the traversal of
the MIME structure.