should we permit `get_completion_signatures<Sndr,Env>()` to return a subset of `get_completion_signatures<Sndr>()`

[ericniebler]

question came from Tomas. for a non-dependent sender, get_completion_signatures<Sndr>() returns some set of completions. if we later ask with an environment like get_completion_signatures<Sndr, Env>(), it is required to return the same type.

but could we permit it to return a subset of the signatures returned from get_completion_signatures<Sndr>(). i don't see why not, but i also can't think of when i would want to do that.