improve serialization for elliptic curve points in the pasta curves

[mimoo]

Once https://github.com/o1-labs/proof-systems/issues/980 is in, we can override the serialization for the pasta curves.

Currently the serialization of a point compressed it to a single x coordinate (so 32 bytes) and add an extra byte for flags where:

• 1 << 7 is set if the y coordinate is greater than -y (as bigints)
• 1 << 6 is set if the point is the point at infinity

For the pasta curves the canonical serialization is usually over 32 bytes and encoded as such:

• 1 << 7 is set in the last byte if the y coordinate is odd (LSB is 1)
• all zero ([0u8; 32]) if it's the point at infinity

This would effectively save 1 byte for every point we serialize, but not only that, it would also be compatible with other implementations implementing this canonical serialization

[mimoo]

One example: https://github.com/arkworks-rs/curves/pull/129/files

[imikushin]

I'm going to look into this.

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