hyper::Error should distinguish http1 TooLarge from other parse errors

Currently if an http1 message head exceeds the http1_max_buf_size, a hyper::Error is returned where err.is_parse() == true. This is indistinguishable from other errors that httparse might bubble up, which poses a problem for deciding how to respond to these errors. A TooLarge can occur just because of the choices made when setting up the max buffer size (many implementations default to 1MB vs the ~400KB default for hyper), vs. an actually malformed HTTP message which probably indicates a misbehaving server.

Could we add a new method to hyper::Error that distinguishes these? hyper::Error::is_buf_size_exceeded() as a strawman?

No file or test path is named. Start by tracing the http1 parsing path where httparse errors become hyper::Error, then inspect how the TooLarge condition is represented. Define and test a distinction between buffer-size exhaustion and malformed-message parse errors, using the proposed Error method as the target API.