fix: transaction account dedupe and roles

[buffalojoec]

Problem

Currently, if the same account is used twice in an instruction, and twice in the accounts list, Mollusk will not unify the roles of the accounts. For example, consider the following instruction:

1. [ writable ] Account 1
2. [  signer  ] Account 2

If the same key was provided for both accounts 1 & 2, they would be serialized into the VM as separate accounts with the following roles:

{
    account_1: { is_signer: false, is_writable: true }
    account_2: { is_signer: true, is_writable: false }
}

Instead they should both be writable and both be signer.

Summary of Changes

Partially reimplement the transaction compilation used in Message::new_with_blockhash to build out the transaction context for Mollusk invokes.