improve the consistency of naming and qualification (#162)

* sender expressions are now `sndr[0-9]?` or `out_sndr` (not just `s`)
* sender types are `Sndr[0-9]?` or `OutSndr`
* scheduler expressions are `sch[0-9]?`
* scheduler types are `Sch`
* receiver expressions are `rcvr[0-9]?` or `out_rcvr`
* receiver types are `Rcvr[0-9]?` or `OutRcvr`
* environments expressions are `env\d?` and have type `Env\d?`
* remove `std::` qualification on all entities besides `std::move` and
  `std::forward`
* fix places still referring to `get_env` in the `std::` namespace