src: move State off of Request (WIP)

[Fishrock123]

error[E0119]: conflicting implementations of trait `endpoint::Endpoint<_>`:
  --> src\endpoint.rs:70:1
   |
54 | / impl<State, F, Fut, Res> Endpoint<State> for F
55 | | where
56 | |     State: Send + Sync + 'static,
57 | |     F: Send + Sync + 'static + Fn(Request) -> Fut,
...  |
67 | |     }
68 | | }
   | |_- first implementation here
69 |
70 | / impl<State, F, Fut, Res> Endpoint<State> for F
71 | | where
72 | |     State: Send + Sync + 'static,
73 | |     F: Send + Sync + 'static + Fn(Request, State) -> Fut,
...  |
83 | |     }
84 | | }
   | |_^ conflicting implementation

Not sure I understand this error. F should be a distinct type for both?? Note: the difference is in F and those compiler messages don't show enough lines for that.

[tirr-c]

I guess the compiler thinks F might implement both Fn(Request) -> Fut and Fn(Request, State) -> Fut. You can make such a type with #![feature(fn_traits, unboxed_closures)] by manually implementing Fn and its supertraits.

[yoshuawuyts]

As per #643 (comment) this is not a direction I feel comfortable with, and I would prefer we see #642 through.

[Fishrock123]

Err... this isn't solving the issue that #642 and #644 solve... this is just general ergonomics, imo? Likewise see #643 (comment)

[yoshuawuyts]

One interesting implication of this PR is that it would've made #670 easier to implement; no State means no Default either.