Introduces the YxId and TxHash types. This will ensure type safety for cases requiring to use an id and cases requiring to use a hash.
To quote D665 :
This is the basis of MalFix with naming txhash/txid.
It is quite tricky to ensure no txid is kept where txhash is meant.
As txid will be rare, I propose for safety to fully rename txid to
txhash, eliminating txid usage before reintroducing txid to COutpoint.