Add IntoIterator for &Vec<>

[leighmcculloch]

What

Add IntoIterator for &Vec<>.

Why

To improve the ergonomics of using the soroban_sdk::Vec type.

So that it is possible to do for _ in &vec. So it is possible to iterate a vec without moving it.

Today if a developer would like to iterate a Vec without moving it, they need to write for _ in vec.iter(). This pattern is a little odd because ordinarily a developer just passes &vec to iterate without moving.

Note that iterating &vec will still be somewhat different than it would be to iterate a ref of a std::vec::Vec from the Rust std library. This is because the Rust std library Vec when iterating a vec is iterating refs to items in the vec. When iterating a soroban_sdk::Vec there is no items in memory to ref, rather during iteration each item is communicated across the host-guest interface and must be passed by value as no one is holding onto its value.

[leighmcculloch]

Tests will pass once this issue merges:

• Add __check_auth wasm test vectors and tests #1334