New byte buffer
This new byte buffer implements a simple dynamic buffer. It aims to
* Provide a clean API.
* Convince the compiler to generate good code. In particular it tries
to avoid unnecessary loads and stores which are otherwise generated
due to the C aliasing rules. This is done using the STATIC_ASSUME(X)
macro, which declare !X as unreachable.
* Have the fast path (when the buffer has enough space) completely
inlined to allow the compiler to batch single byte writes to the
buffer into larger stores.