Cleanup the serialization file

[jtraglia]

• Split DeserializeG1Point into DeserializeKZGCommitment and DeserializeKZGProof.
  • This better matches the spec and doesn't require casts anymore.
• Rename DeserializeG1Point to deserializeG1Point (to make private).
• Group constants and types.
  • Move CompressedG1Size & CompressedG2Size to top.
  • This just because I like to group the spec constants together.
• Reimplement DeserializeBlob to better match the spec.
  • It's a lot cleaner now IMO too.
  • Since Blob is a well-defined type, we don't need to check it's length.