Previously, we weren't too consistent about the definition of frame and
sample when it relates to raw audio data. This brings all the usages in
the context of raw data in line (hopefully), with samples referring to
a single PCM value, and frames referring to the multiple samples that
make up an instant's audio across all channels.
This will allow reuse of the allocations, instead of reallocating a
FixedArray for every block that changes size. Generally, it will be
possible to reuse AudioBlock memory throughout most of the pipeline
at least while in a single process.
