WIP: Allow subclassing MultipartSegment

[defnull]

Allow use of the core parsing functionality for other use cases (e.g. multipart/byterange as described in #73) by allowing users to subclass MultipartSegment.

This is a WIP. First step was to clean up the API between PushMultipartParser and MultipartSegment to make it easier to hook into those APIs. The new MultipartSegment._on_* methods are still private APIs and may change between releases, so be careful. But it looks nice now, so future changes should be less likely.

One idea was to split the segment class into MultipartSegmentBase implementing universal parsing logic only, and MultipartSegment implementing the additional checks and logic for form-data segments. I tried that, but it reduced throughput by up to 5% in some tests and made documentation more complicated. I'm not sure if I really want that, but it's not from the table just yet.