Manually implement it for Wtxid, Txid and BlockHash, where the all-zero
"hash" has a consensus meaning. But in general we should not be
implementing this method unless we have a good reason to do so. It can
be emulated or implemeted in terms of from_byte_array.
The use of Wtxid::all_zeros is obscure and specific enough that I am
tempted to drop it. But for txid and blockhash, the 0 hash appears in
actual blockdata and we should keep it.
All other uses of all_zeros were either in test code or in places where
the specific hash was not important and [u8; 32] was a more appropriate
type.
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