Hardcode error type in IntoVisitor

[jsdw]

This PR hardcodes the error type, because, in a nutshell, we run into this issue in some places otherwise:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=51e845f1f73ed72b0eb9d3d29172089a

We can avoid that specific issue by not having any bounds on DecodeAsType and instaed only bounding implementations of DecodeAsType, but that just means that DecodeAsType doesn't imply IntoVisitor, and so code like the following fails:

#[derive(DecodeAsType)]
#[decode_as_type(trait_bounds = "T: DecodeAsType")]
struct Foo<T> {
    val: Option<T>
}

This fails because for Option<T> to implement DecodeAsType, it must implement IntoVisitor, and it only does so if T: IntoVisitor, T::Visitor::Error: Into<Error>, ie we can't just use DecodeAsType as a trait bound.

With a hardcoded Error type on IntoVisitor, DecodeAsType can imply IntoVisitor, and the above issue goes away. I also added a VisitorWithCrateError adapter struct which makes any visitor whose error is convertible into crate::Error into a visitor returning crate::Error, so that it's easy to still adapt any viable visitor into something that can be used with IntoVisitor.

[tadeohepperle]

Nice, this will allow for all sorts of easy conversions.

[jsdw]

Yeah; it basically replaces the "auto conversion" we did before but has the nice effect of simplifying the code anyway and making DecodeAsType work better, so I'm quite happy with this solution in the end :)

[niklasad1]

LGTM

[lexnv]

Nice one!