Refactor form parsing to be sans-io

[davidism]

Currently, the parser assumes that it's reading from an open stream. However, in ASGI the data is fed as events, there is no input stream available. "Sans-IO" means writing the parser as a self-contained state machine that can be fed data incrementally with function calls rather than by passing a file. Some higher layer would do the IO and pass it to the parser, and the parser would be able to operate no matter how that works.

@edk0 created a proof-of-concept in #1330, but that code is not documented and pretty complicated. I'd prefer we treat that as a first pass towards a well tested, maintainable implementation.

[davidism]

@pgjones assigning this to you after our chat. It's not part of the 2.0.0 milestone, no rush. Here's the start of my comments I was referring to about extra multipart headers: #1848 (comment)