refactor(spl): use assume to replace cheatcode_set_*

[Stevengre]

This optimization improves

1. the proving performance of test_process_get_account_data_size by about 34% (1.5x faster), reducing the runtime from 3748 s to 2471 s;
2. the structure of the proof from multiple branches into one.

[Stevengre]

The proof before and after stuck on the same

function: std::cell::RefCell::<&mut [u8]>::try_borrow
span: /rust/library/core/src/cell.rs:545

[jberthold]

LGTM.
This supersedes #70 because it includes the submodule update.