Module buffer

An encapsulation of BufReader's buffer management logic.

This module factors out the basic functionality of BufReader in order to protect two core invariants:

• filled bytes of buf are always initialized
• pos is always <= filled Since this module encapsulates the buffer management logic, we can ensure that the range pos..filled is always a valid index into the initialized region of the buffer. This means that user code which wants to do reads from a BufReader via buffer + consume can do so without encountering any runtime bounds checks.