Similar to speeding up serialization; in our parsing logic we did a
bunch of expensive stuff then expensively inverted it. Drop everything
except the essential checks and then memcpy.
`secp256k1_pedersen_commit_serialize` would call `_load` (which does a
sqrt to fully decompress the key, then a conditional negation based on
the flag), then check the Jacobian symbol of the resulting y-coordinate,
then re-serialize based on this.
Instead, don't do any of this stuff. Copy the flag directly out of the
internal representation and copy the x-coordinate directly out of the
internal representation.
Checked that none of the other _serialize methods in the modules do
this.
Fixes#293
The expression `G.decode('hex')` fails with the following error message
on Sage 9.5:
AttributeError: 'str' object has no attribute 'decode'
Fix that by converting the hex-string to bytes using `bytes.fromhex`.
The previous implementation returns an off-curve point for the input t=0.
This rewrite addresses that issue by implicity returning the on-curve point
(d, sqrt(1 + b)), which is the point that the paper Indifferentiable Hashing
to Barreto–Naehrig Curves suggests returning in this case.
Note: At the moment it is cryptographically impossible for the input t to be 0.
Silence a compiler warning about an unitialized use of a scalar in case
the user tries to provide a 0-length list of commitments.
Also ensures that commitments have normalized field elements when they
are loaded into ges.
