starlark-rust

Summary This PR makes three Bazel-compatible improvements to Starlark built-in methods: 1. * — Add optional and parameters, matching the Starlark spec and Python semantics. The substring is tested instead of the full string. 2. — Same as above for . 3. — Allow to be passed as a named keyword argument (e.g., ), not just positional. This matches Bazel's Java Starlark implementation and Python's . Motivation These changes are needed for Bazel compatibility. Real-world Bazel BUILD files and macros use these features extensively: Related Issues bazelbuild/starlark#56 — The Starlark spec was updated to include / params for /, but never implemented them. Conformance test suite inconsistency* — The bazelbuild/starlark conformance tests document this with: Implementation Details / Reuses the existing infrastructure (already used by , , , , ) to slice the string before testing. Supports both single string and tuple-of-strings prefix/suffix arguments. Removes from the parameter, allowing it to be passed as either positional or named. The parameter remains positional-only. Testing All 981 existing tests pass (828 lib + 153 doc) New tests added for each change: — 7 assertions covering basic, negative indices, and tuple prefix — 4 assertions covering basic, negative indices, and tuple suffix — 3 assertions covering missing key, existing key, and fallback value