fix: ensure correct signature length

[xoudini]

Switches the ECDSA signer to use BugNum::to_vec_padded to ensure the signature is twice the component length for each curve.

Fixes #1 (as far as I can tell).

[xoudini]

The same issue exists in Jwk::try_from:

lers/src/api/jws.rs

Lines 122 to 123 in 991a275

	x: BASE64.encode(x.to_vec()),
	y: BASE64.encode(y.to_vec()),

Every now and then when either BigNum ends up with 8 (or more) leading zero bits, this error is produced:

Parse error reading JWS: failed to unmarshal JWK: go-jose/go-jose: invalid EC public key, wrong length for x: \"{\\\"crv\\\":\\\"P-256\\\",\\\"kty\\\":\\\"EC\\\",\\\"x\\\":\\\"rdYu9rs4jFRTSPIUiUryoHNu_j32BLaLEK3ZzVLc9w\\\",\\\"y\\\":\\\"eAgyilCBWz-FUUXdhpOPmBxdvbWsphsT-ep-paCx8-k\\\"}

With some formatting of the data:

{
  "crv": "P-256",
  "kty": "EC",
  "x": "rdYu9rs4jFRTSPIUiUryoHNu_j32BLaLEK3ZzVLc9w",
  "y": "eAgyilCBWz-FUUXdhpOPmBxdvbWsphsT-ep-paCx8-k"
}

...where x turns out to be 31 bytes instead of the expected 32.