Enforce recursion limits

[duesee]

Hey, cargo-fuzz revealed a stack overflow in my implementation of search-keyand thus I had a look at which parsers are defined recursively.
You can see the result here: duesee/imap-codec#3

I am leaving this issue here as an reminder that you probably want to introduce a recursion limit to the body parser.

[djc]

Thanks for the reminder! I probably won't have time to implement this any time soon, but would be grateful if someone else could pick it up.