Get system accounts dynamically

[moshthepitt]

See: https://github.com/solana-labs/solana/blob/a1a0d6f84b30ecea1fd1b699aa3cd6ab2741db77/programs/bpf/rust/sysvar/src/lib.rs#L32

[moshthepitt]

Can we use the Fees sysvar to dynamically set merchant fee levels during registration?

[moshthepitt]

Done for Clock in #21