Better filesystem serialization

[samoht]

Would be great if instead of using s-expression the messages where serialised a sub-trees in the filesystem. Would be easier to see what have changed.

[gregtatcam]

email messages are append and delete only, they can not be modified.
the metadata related to mailboxes and messages can be edited
but all data is going to be encrypted so the changes can only be observed in general, i.e. that a metadata changed without the knowledge of what actually changed.