provide bulk term deletion API to remove multiple terms from a document

[bremner]

In an email discussion about improving the performance of _notmuch_message_remove_terms, Olly had two suggestions:

A variant of remove_term() which took an iterator could avoid [looking strings up] (and automatically increment the iterator for you).

We could also/instead provide a method to directly remove all terms with a specified prefix (or maybe between two specified terms) from the document, which is probably better for this specific case as I think it can be done even more efficiently.