Drop Vec elements on scope exit and infer Option::None from call arguments (v0.1.19)

[Cod-e-Codes]

Summary

• Codegen: Vec<T> scope-exit drop now drops remaining elements when T needs destruction, then ion_vec_free. This impacts any Vec<String>, Vec<Vec<U>>, or Vec of structs/enums with owned fields (previously the backing array was freed and elements leaked). Vec::get of such T hollows the slot (already specified as move-out). Vec::set drops the previous element. Vec<int> and other Copy elements are unchanged. Box<T> drops T before ion_box_free when T needs destruction; Box::unwrap still does not drop T.
• Type checker: unannotated Option::None / other no-payload generic variants infer T from a call argument's parameter type (any position), matching return and struct-field positions. Unannotated let empty = Option::None still requires an annotation. This impacts passing Option::None or Result::Err(...) directly into a function.

Test plan

• cargo fmt --check
• cargo test
• cargo clippy -- -D warnings
• tests/test_runner.sh (Linux CI)
• Example ion-build projects compile
• Confirm Vec/Box string scope-drop tests plus LSan rows
• Confirm take(Option::None) infers T; unannotated let empty = Option::None still errors