Uses the go module's `replace` directive to delegate keccak computation
to precompiles.
This is still in draft because it needs more testing. Also, it relies on
a PR that I created, that hasn't been merged yet.
_Note that this PR doesn't implement the stateful keccak state
structure, and it reverts to the current behavior. This is a bit silly
since this is what is used in the tree root computation. The runtime
doesn't currently export the sponge. I will see if I can fix that in a
further PR, but it is going to take more time. In the meantime, this is
a useful first step_
