implement Stack::get and Index for #14

[kreinba]

Closes #14.

Adds Stack::get(&self, idx) -> Option<&V> and a corresponding Index<usize> implementation for Stack, mirroring the behavior of Vec::get and Vec's Index impl: get returns None when the index falls outside the populated range [0, len), while Index panics on the same condition with a message naming the index and the current length. The new public surface lives in a fresh src/access.rs module wired into src/lib.rs.

Verified locally with cargo test --all-features (29 unit tests pass, 2 doc-tests pass), cargo test --release --all-features, cargo fmt --check, cargo doc --no-deps, and cargo clippy --all-features -- --no-deps (no new warnings introduced; the seven pre-existing warnings on master are unchanged).