A rewrite of multipart library.

1. Support streaming in the simple case.  Put huge warnings around the
buffering API.
2. Support nested multipart messages.
3. Support various kinds of multipart messages, including all the various
absurdities of multipart email messages.
4. Error out on various invalid types of data that might come through.
5. Gracefully several edge cases that occur when the chunk size is smaller
than the boundary length, or cuts into the \r\n, etc.
6. Provide both positive and negative tests.