extract packet crate

[kevinheavey]

Problem

solana_sdk::packet imposes a solana_sdk dependency on solana_client

Summary of Changes

• Move to its own crate
• Re-export with deprecation
• Make serde and bincode optional in the new crate. This involves adding a new dependency cfg_eval as advised by the serde_with docs: https://docs.rs/serde_with/latest/serde_with/guide/serde_as/index.html#gating-serde_as-on-features