Program: add custom error codes for readonly writes

[buffalojoec]

Problem

Working more with the Firedancer conformance harness, yet another inconsequential mismatch between the BPF version and its original builtin version has popped up.

When a builtin program attempts to write to an executable or read-only account, it will be immediately rejected by the TransactionContext. However, BPF programs do not query the TransactionContext for the ability to perform a write. Instead, they perform writes at-will, and the loader will inspect the serialized account memory region for any account update violations after the VM has completed execution.

The loader's inspection will catch any unauthorized modifications, however, when the exact same data is written to the account, thus rendering the serialized account state unchanged, the program succeeds.

Summary of Changes

In order to maximize backwards compatibility between the BPF version and its original builtin, we add these checks from TransactionContext to the program directly, to throw even when the data being written is the same as what's currently in the account.

Unfortunately, the two InstructionError variants thrown do not have ProgramError counterparts, so we mock them out with custom error codes.

[joncinque]

Looks good overall, just a question on the executable check

[joncinque]

Looks great!