bip-0375: check test vector signatures in the test runner #2317

The P2WPKH inputs of the test vectors spend a witness program equal to
SHA256(pubkey)[:20] instead of HASH160(pubkey), so the key each input
carries cannot spend it. None of the stored ECDSA signatures verifies
against the transaction its PSBT describes, whatever the input type. The
reference validator does not check signatures, so no test fails.

Set every P2WPKH witness program to HASH160 of the input's public key,
in PSBT_IN_WITNESS_UTXO and in the supplementary witness_utxo and
prevout_scriptpubkey. Re-sign the PSBT_IN_PARTIAL_SIG signatures
deterministically (RFC 6979, low S): BIP 143 for P2WPKH and P2SH-P2WPKH,
the legacy signature hash for P2PKH and for the bare OP_2 input. Where
an input sets PSBT_IN_SIGHASH_TYPE, its signature uses that type, as BIP
174 requires.

Two vectors carried a signature with no defined signature hash: one on
a segwit v2 input, one in the vector whose output lacks PSBT_OUT_SCRIPT.
Remove both signatures, and mark the segwit v2 input as not signed in
the supplementary material. Both vectors still fail for their stated
reason.

Inside the PSBTs only PSBT_IN_WITNESS_UTXO and PSBT_IN_PARTIAL_SIG
change, and the test runner's result for every vector is unchanged.

The validator does not read PSBT_IN_PARTIAL_SIG. Nothing checks that a
P2PKH, P2WPKH or P2SH-P2WPKH program commits to its signing key. So the
wrong programs and signatures that the parent commit corrects pass the
test runner.

Add signatures.py. The test runner applies it to every vector. It
checks that:
- a single-key program commits to the keys in PSBT_IN_PARTIAL_SIG and
  PSBT_IN_BIP32_DERIVATION;
- a P2SH program commits to PSBT_IN_REDEEM_SCRIPT;
- a signature's sighash byte equals PSBT_IN_SIGHASH_TYPE, if set;
- each signature verifies: BIP 143 for P2WPKH and P2SH-P2WPKH, the
  legacy sighash for P2PKH and bare scripts.

A signature the check cannot verify is a failure. The validator is
unchanged.

hash160 uses the pure Python RIPEMD-160 from bip-0352. hashlib lacks
RIPEMD-160 on some OpenSSL builds.