Support _variant in outer level enum formatting for Display

[JelteF]

Resolves #142
Resolves #239

Synopsis

This adds back support for top-level format strings of the Display derive. It
now includes the display of the variant whenever the _variant placeholder
apears be found. It also supports using the field

Solution

This does not include the same support for Debug, since it is considered much less
useful there and the differences between the Debug implementation and Display
implementation make it a non-trivial port.

Only named arguments are supported in the format string, no positional ones.
This made the implementation easier, maybe in a future PR positional support
can be added.

This bumps MSRV to 1.70.0, on earlier versions the derived code throws the following error:

error: there is no argument named `_0`
    --> tests/display.rs:1373:26
     |
1373 |                 #[derive(Display)]
     |                          ^^^^^^^
     |
     = note: did you intend to capture a variable `_0` from the surrounding scope?
     = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
     = note: this error originates in the derive macro `Display` (in Nightly builds, run with -Z macro-backtrace for more info)

Checklist

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

[JelteF]

Still needs some cleanup, error-ing on edge cases, and updated docs, but the general design seems to work.

[JelteF]

Still needs some cleanup, error-ing on edge cases, and updated docs, but the general design seems to work.

Okay all done now, this is ready for review.