* Add credit-only flag to AccountMeta, default to false
* Sort keys by is_credit_only within signed/unsigned groupings
* Process and de-dupe program keys along with other account keys
* Add message helper functions
* Fix test
* Improve comment
* s/is_credit_only/is_debitable
* Add InstructionKeys helper struct, and simplify program_position method
* Add num_readonly_accounts slice
* Impl programs in account_keys
* Emulate current account-loading functionality using program-account_keys (breaks exchange_program_api tests)
* Fix test
* Add temporary exchange faucet id
* Update chacha golden
* Split num_credit_only_accounts into separate fields
* Improve readability
* Move message field constants into Message
* Add MessageHeader struct and fixup comments
Added CompiledInstruction::program_id() so that we don't need to pass
around instruction indexes just for Message::program_id().
Also added Message.program_ids() that returns a slice so that we
can move those pubkeys into Message::account_keys.
Rob Walker
Tyera Eulberg
Greg Fitzgerald
Sathish Ambley