Refactor get_* calls in harnesses for fewer steps (#88)

This PR refactors calls to `get_{account, mint, multisig}` where
possible in all proof harnesses to reduce the amount of steps proofs
need to take. A selection of proofs were run again to see the reduction
in steps, not every harness could be refactored and some proofs go for
long without stopping so it is hard to see how the count changes.

The naming convention introduced is inspired by specification languages
that have an `old(...)` operator to talk about pre-strate (e.g. Dafny).
In the post state I added `new` to variable so it would be clear this is
captured after the implementation has executed.

Proofs run and change in steps:
- initialize_account: 89207 -> 89207 = no change, probably indicates not
getting into the else branch
- transfer: 4011 -> 3459 = 14% reduction
- mint_to: 3248 -> 2834 = 13% reduction
- burn: 4699 -> 3457 = 26% reduction
- close_account: 3711 -> 3021 = 19% reduction
- transfer_checked: 4755 -> 4065 = 15% reduction
- burn_checked: 5290 -> 3910 = 26% reduction
- approve: 3049 -> 2773 = 9% reduction
- approve_checked: 3793 -> 3517 = 7% reduction
- revoke: 2409 -> 2133 = 11% reduction
- freeze_account: 3199 -> 2647 = 17% reduction
- thaw_account: 3199 -> 2647 = 17% reduction
- mint_to_checked: 3689 -> 3275 = 11% reduction
- sync_native: 2331 -> 2055 = 12% reduction

The concrete test harnesses that are able to run with our restricted
harnesses were also run successfully

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Co-authored-by: automergerpr-permission-manager[bot] <190534181+automergerpr-permission-manager[bot]@users.noreply.github.com>

Author: dkcumming