Adopt stdexec's concept of environments and queries

[ispeters]

This gives us better standards conformance and dependently-typed Senders; it kinda looks like we need this to be able to adapt sequence-senders, too, which something I want to explore.

We'll need a migration path from the current way (with, for example, unifex::sender_traits<>) to the new way. I'm not sure, yet, what that entails, but it's at least:

• a way to synthesize a unifex::sender_traits<S> for some S that expresses its properties the new way;a nd
• a way to respond correctly to the new environment-based queries for a type S that declares its sender_traits<>.