Adds rpc/examples/createMint demonstrating the idiomatic way to create
a new SPL Token mint — the two-instruction composition readers have
historically had to reconstruct themselves:

  1. system.CreateAccount allocates 82 bytes funded to the rent-exempt
     minimum, owned by token.ProgramID.
  2. token.InitializeMint2 writes the mint state (decimals, mint
     authority, freeze authority). Mint2 is preferred over Mint because
     Token v1.1+ dropped the Rent sysvar the original required.

The example runs end-to-end on devnet: generate payer + mint keypair,
airdrop, fetch rent via GetMinimumBalanceForRentExemption, build both
instructions, sign with both keypairs (the mint keypair authorizes
creation at its own pubkey), and SendTransaction. Inline comments
point to the Token-2022 variant and note where extensions would slot
in before InitializeMint2.

Closes solana-foundation#277