lib: add stream_discard function

[LPardue]

QUIC requires that stream data is presented to receivers as a contiguous byte
stream. The existing stream_recv() function allows applications to read
contiguous bytes from receive streams, copying data into an application-provided
slice.

Applications may have cases where they want to skip over data in a receive
stream without using it. stream_recv() could be used in these cases, at the cost
of copying.

This change adds a stream_discard function that acts similarly to stream_recv.
Callers indicate a maximum size of bytes to discard, and up to that many will be
discarded without a copy. The return values of this function are identical to
stream_recv, meaning that existing business logic and stream state management
can remain the same. Application can use either function interchangeably,
depending on their needs.

[jannes]

i just wonder whether it's necessary to parameterize all those tests on emit/discard instead of some isolated discard tests

[LPardue]

That's a good question. Many of the tests are intended to stress stream states or exercise flow control/congestion congestion as a result of taking data from streams. There's 60+ tests that were changed as part of this PR, I can't confidently tell if if its safe to not test the change if I don't test it. Future refactors may also cause unexpected breakages.

Given how critical streams are, and how many edge cases (or bugs) we've had to accommodate for, I don't really want to take risks.