Example of bazel-rust workspace

Currentl problem is that the optional dependencies are not fetched. These depenedencies neither are provided by all_crate_deps nor can be specified manually.

The //rust/gui:gui_bin target can be built in two versions:

1. defaul one, no extra features. No optional dependencies needed.
2. debug one, it is enabled by "sdl2" cargo feature. This requires extra dependencies the bazel switches between the features by --//rust/gui:use_sdl flag.