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

Except in the In Progress vector added in 0.1.4, 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 other vectors' 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. 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.

No signature hash is defined for segwit v2. The previous commit removed
the same signature from the other segwit v2 vector. The validator still
rejects the input for its segwit version.

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.

An input whose PSBT lacks its spent output, or a P2SH input without
PSBT_IN_REDEEM_SCRIPT, gives no signature hash to check against. Such a
PSBT is malformed, and rejecting it is the validator's job, so the
signature check skips the input instead of failing the vector.

The existing vectors only show that valid signatures pass. If the
signature check stopped verifying, every vector would still pass.

Flip the low bit of the last byte of s in the first PSBT_IN_PARTIAL_SIG
of the valid vectors, keeping the DER encoding and sighash byte valid,
and require check_partial_signatures to report a failure.