constraints on into_variant's get-state function are incorrect #314

For a queryable object `env`, `FWD-ENV(env)` is an expression whose
type satisfies `queryable` such that for a query object `q` and a pack
of subexpressions `as`, the expression `FWD-ENV(env).query(q, as...)`
is ill-formed if `forwarding_query(q)` is `false`; otherwise, it is
expression-equivalent to `env.query(q, as...)`.
+The type `FWD-ENV-T(Env)` is `decltype(FWD-ENV(declval<Env>()))`.

The member `impls-for<into_variant_t>​::​get-state` is initialized with
a callable object equivalent to the following lambda:

  []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept
-   -> type_identity<value_types_of_t<child-type<Sndr>, env_of_t<Rcvr>>> {
+   -> type_identity<value_types_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>> {
    return {};
  }