Previously the buzhash boundary checker used a single value for the
window size, both as the buzhash buffer size when constructing a hash
object, and reported as its window size to the boundary checker
interface. This was wrong because we don't always pass single byte
values to the hasher, for example refs are 20 bytes.
The compound list chunking compensated for this by only passing the
first byte of each list leaf's ref rather than the full ref. This is bad
because there is obviously less entropy in 1 byte vs 20 bytes.
The meta sequence chunking compensated for this by multiplying the
chunking window size by 20, but this also had the effect of
unnecessarily considering 20 times more chunked elements than would fit
in the buzhash buffer.
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