program: adjust account load ordering to match builtin

[buffalojoec]

Problem

Working with the Firedancer tooling some more, I realized that - in many cases - the builtin version of ALT will load an account and then run one or more checks on it before moving onto loading the next one.

In the BPF version, we follow the common BPF program pattern of loading all accounts up front with subsequent calls to next_account_info. However, this can cause ProgramError::NotEnoughAccountKeys in situations where the builtin may have thrown a different error (such as InstructionError::IncorrectOwner).

Summary

Change the ordering of account loads and checks to exactly match the builtin version.

[joncinque]

Looks great!