depset vs. starlark set: add conversion methods and documentation about which one to use

[tetromino]

Now that Bazel has sets as a core data structure in the Starlark language, we ought to

• document why both depsets and Starlark sets exist, compare their performance characteristics, and explain which one you should use in which case
• add a to_set (or maybe to_mutable_set?) method to depset objects
• optimize the depset constructor when constructing from a starlark set rather than an arbitrary sequence value

[tetromino]

Note that depset.to_set() would require --incompatible_always_check_depset_elements to be enabled (which is the case by default)