Update code for compatibility to Bazel 0.27 (#232)
Update code for compatibility to Bazel 0.27 (#232)

There are two changes in Bazel 0.27:
 - .libraries_to_link will return a depset
 - direct iteration over depset will be forbidden

For compatibility, we add a (temporary) type test.
bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust