The work on this was started last week while I was waiting
for a decision on the "Async Snappy" PR. It was prompted by
a failing test in the test suite, where the HashingStream
was inserting some incorrectly padded chunks that affected
the result of `hash_tree_root`. Instead of working around
the problem in the HashingStream, I've decided to implement
a planned optimisation that allows us to remove the hashing
stream altogether.
With the optimisation in place, `hash_tree_root` will now
use only stack memory and only the precise amount neccesary
to build the chunks-merging tree.
3ecb197635