Fix incorrect fmt::Pointer implementations attempt two

[JelteF]

Resolves #328
Requires #377
Requires #380

Synopsis

Debug and Display derives allow referring fields via short syntax (_0 for unnamed fields and name for named fields):

#[derive(Display)]
#[display("{_0:o}")]
struct OctalInt(i32);

The way this works is by introducing a local binding in the macro expansion:

let _0 = &self.0;

This, however, introduces double pointer indirection. For most of the fmt traits, this is totally OK. However, the fmt::Pointer is sensitive to that:

#[derive(Display)]
#[display("--> {_0:p}")]
struct Int(&'static i32);

// expands to
impl fmt::Display for Int {
    fn fmt(&self, f: fmt::Formatter<'_>) -> fmt::Result {
        let _0 = &self.0; // has `&&i32` type, not `&i32`
        write!(f, "--> {_0:p}") // so, prints address of the `_0` local binding, 
                                // not the one of the `self.0` field as we would expect
    }
}

Solution

Pass all local bindings also as named parameters and dereference them there.
This allows "{_0:p}" to work as expected. Positional arguments and expressions
still have the previous behaviour. This seems okay IMHO, as we can explain
that in expressions these local bindings are references and that you need
to dereference when needed, such as for Pointer.

A downside of the current implementation is that users cannot use the names of our named parameters as names for their own named parameters, because we already use them. With some additional code this is fixable, but it doesn't seem important enough to fix. People can simply use a different name when creating their own named parameters, which is a good idea anyway because it will be less confusing to any reader of the code. If it turns out to be important to support this after all, we can still start to support it in a backwards compatible way (because now it causes a compilation failure).

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[tyranron]

@JelteF could you update this PR to the latest master? I tried, but there are some non-trivial conflicts bound to older implementation of #377.

[JelteF]

Rebased now, there's still a few more tests I want to add though.

[JelteF]

Okay it turned out that the tests I wanted to add already existed, so this is ready for review again.

[tyranron]

I've extracted the similar code to the parent module and implemented the 2nd suggestion regarding expansion.

LGTM for me now, and I'm ready to merge. Feel free to leave any notices if you still have ones.