[bls-signatures] Refactor pubkey, signature, and proof of possession modules

[samkim-crypto]

Problem

As I was tackling #474, I realized that each of the pubkey.rs, signature.rs, and proof_of_possession.rs files are becoming quite long and harder to read.

Summary of Changes

In preparation for adding the new PubkeyAffine, SignatureAffine, and ProofOfPossesionAffine types, I first did a refactor of the pubkey.rs, signature.rs, and proof_of_possession.rs types.

Each each pubkey, signature, and proof_of_possession modules are divided into

• points.rs: contains the Projective type and will soon host the Affine type
• bytes.rs: contains the serialized types like Pubkey (soon to be PubkeyUncompressed) and PubkeyCompressed.
• conversion.rs: contains any conversion logic between points