Merge bitcoin/bitcoin#23325: mempool: delete exists(uint256) function

4307849256761fe2440d82bbec892d0e8e6b4dd4 [mempool] delete exists(uint256) function (glozow)
d50fbd4c5b4bc72415854d582cedf94541a46983 create explicit GenTxid::{Txid, Wtxid} ctors (glozow)

Pull request description:

  We use the same type for txids and wtxids, `uint256`. In places where we want the ability to pass either one, we distinguish them using `GenTxid`.

  The (overloaded) `CTxMemPool::exists()` function is defined as follows:
  ```c
  bool exists(const uint256& txid) const { return exists(GenTxid{false, txid}); }
  ```
  It always assumes that a uint256 is a txid, which is a footgunny interface.
  Querying by wtxid returns a false negative if the transaction has a witness. :bug:

  Another approach would be to try both:
  ```c
  bool exists(const uint256& txid) const { return exists(GenTxid{false, txid}) || exists(GenTxid{false, txid}); }
  ```
  But that's slower and wrongfully placing the burden on the callee; the caller always knows whether the hash is a txid or a wtxid.

ACKs for top commit:
  laanwj:
    Code review ACK 4307849256761fe2440d82bbec892d0e8e6b4dd4
  jnewbery:
    Tested and code review ACK 4307849256761fe2440d82bbec892d0e8e6b4dd4
  MarcoFalke:
    review ACK 4307849256761fe2440d82bbec892d0e8e6b4dd4 👘

Tree-SHA512: 8ed167a96f3124b6c14e41073c8358658114ce121a15a4cca2db7a5ac565903a6236e34e88ac03382b8bb8b68e3999abbfc5718bc8c22476554d6b49a5298eec

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