Adopt stdexec's concept of environments and queries

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` 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<>`.