Skylark type hints support.

[taras....@gmail.com]

Is someone working on adding type hints (https://www.python.org/dev/peps/pep-0484/) and/or syntax for variable annotations (https://www.python.org/dev/peps/pep-0526/). Ideally Skylark macros and extensions should be as simple as possible, but in practice (https://github.com/bazelbuild/rules_apple/tree/master/apple/bundling) they end up being fairly complex reflecting the domain complexity.

Having type hints would greatly improve ability to understand existing and write new extensions. Whether this information would be used by the interpreter (probably not due to extra overhead), external tool like Skylint or something else can be discussed separately.