Make Explicit or Better Document seq! Reffering Behavior

[bbb651]

(moved from #406)

I'm just starting out with winnow and I've spent an embarrassingly long amount of time getting unreadable type error from the compiler because I didn't know about the behavior of referring to other struct fields and it doesn't seem to be documented anywhere.
My code was something like:

seq! { Foo {
    identifier: identifier,
    // ...
}}

and I was trying to use identifier thinking it was the parser and not the field. Not sure if it's possible with macro_rules! or worth it but having it prefixed with let or replacing : with = to explicitly have these references could really help, but at the very least document this properly.

[epage]

seq! is trying to mirror the struct initialization syntax within Rust today which is why : is used. I hadn't thought of shadowing issues like this that arise from the discrepancy of behavior.

At the moment, my feeling is there is no good answer and I'd prefer to keep what we have.

I have posted #535 to see what tweaks can be made to clarify what is happening