This PR replaces nonces in two places with pre-salted hashers.

The nonce is chosen to be 64 bytes long so that it forces the SHA256
hasher to process the chunk. This leaves the next 64 (or 56 depending if
final chunk) open for data. In the case of the script execution cache,
this does not make a big performance improvement because the nonce was
already properly padded to fit into one buffer, but does make the code a
little simpler. In the case of the sig cache, this should reduce the
hashing overhead slightly because we are less likely to need an
additional processing step.

ninja all check-all
ninja bench-bitcoin