Add `work set token' to backend interface

[lifflander]

Independent of the exact implementation of concurrent work sets, the first stage to augmenting the current backend interface with grouping information across create_concurrent_work<A>(...) invocations is the work set token.

A work set token is a backend defined type: types::work_set_token_t

Before a token is used, it is registered with a call to the backend:

types::work_set_token_t register_work_set();

This token can be added to the interface of task, task_collection, use, and (possibly?) handle.

types::work_set_token_t const& get_work_set_token() const;