Extract signer crates

[kevinheavey]

Problem

solana_sdk::signer needs to be made available outside solana_sdk

Summary of Changes

• Rip out the following crates (not putting them in one crate because each one has quite significant dependencies): solana-presigner, solana-seed-derivable, solana-seed-phrase, solana-signer
• Make the keypair module optional in solana-signer. This didn't go in its own crate because it seems it's not possible to do so without breaking the Signers trait
• replace serde_json usage in keypair.rs with a simple inline parser
• remove itertools from the unique_signers function

This branches off #2295 so that needs to be merged first