Bug info about demux_enum

[neurusL]

Hi, we are a team from Carnegie Mellon University using Hydroflow for reactive programming. The idea Hydroflow representing code as dataflow is really inspiring and convenient for us to use. Thanks for your great project!
A slight issue we encounter is when we use the operator demux_enum, the debug info auto-generated by rust compiler is a bit confusing. For example, we didn't notice there's a requirement for exhaustive matching for succeeding dataflow of demux_enum, while the error message is, for example:

   --> src/server.rs:33:29
   |
33 |       let mut hf: Hydroflow = hydroflow_syntax! {
   |  _____________________________^
34 | |         // Define shared inbound and outbound channels
35 | |         outbound_chan = union() -> dest_sink_serde(outbound);
36 | |         inbound_chan = source_stream_serde(inbound)
...  |
72 | |      
73 | |     };
   | |_____^ expected `(_, ())`, found `()`
   |
   = note:  expected tuple `(_, ())`
           found unit type `()`

Another example is when succeeding dataflow's input type doesn't match demux_enum's output type, the error message refers to demux_enum rather than succeeding dataflow, for example:

  --> src/server.rs:39:16
   |
39 |             -> demux_enum::<MessageWithAddr>();
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `(SocketAddr,)`, found `()`
   |
   = note:  expected tuple `(std::net::SocketAddr,)`
           found unit type `()`
   = note: required for `(impl Pusherator<Item = ()>, (_, ()))` to implement `PusheratorListForItems<((std::net::SocketAddr,), ((std::net::SocketAddr,), ()))>`
   = note: 3 redundant requirements hidden
   = note: required for `(impl Pusherator<Item = (SocketAddr,)>, (impl Pusherator<Item = (SocketAddr,)>, (..., ...)))` to implement `PusheratorListForItems<((std::net::SocketAddr,), ((std::net::SocketAddr,), ((std::net::SocketAddr, protocol::Entry), ((std::net::SocketAddr,), ((std::net::SocketAddr,), ())))))>`

We feel like maybe in future sprints, you can improve these bug reports to make Hydroflow more user friendly. Thanks!

[MingweiSamuel]

Will simplify the implementation a bit. Hands a bit tied by the way macros work, but should be a bit better #1204

[MingweiSamuel]

Can't really make the error messages perfect due to how proc macros work but they should be a lot better now. Feel free to post any weird error messages you get and we can see if it can be improved further

[neurusL]

Thanks a lot! It's indeed more readable now.