Add record! macro for making labelled HLlists

[lloydmeta]

https://github.com/lloydmeta/frunk/pull/124/files/7e4bc32eeac38288f9cedf60c5e5ab68c54e3f30#r220041930

Proposal

I wonder if we should add a macro:

record! { // bikeshed
    name: "joe",
    age: 3,
    is_admin: true,
}

Possibly also:

Record! { // bikeshed
    name: &'static str,
    age: i32,
    is_admin: bool,
}

[ExpHP]

This looks like the perfect use case for function procedural macros, which IIRC are being stabilized in this release cycle?

[ExpHP]

I may have misinterpreted; I thought the identifiers were interpreted as the field names, but I now see it's possible that they were already assumed to exist as type aliases.

In the latter case, this macro is already possible with macro_rules; but a label! proc macro to generate the field label types would still be cool.

Having Record! support both type field = (f, i, e, l, d); and automatic label type generation can be done using string literals to disambiguate:

Record! {
    age: u32, // uses the existing type `age`
}

Record! {
    "age": u32, // generates a label type
}

(But more is not always better)

[Centril]

The automatic generation of the label type does seem like a win in terms of ergonomics and let's you get away with being more ad-hoc "on the flow".

Perhaps for disambiguation, a leading sigil would be clearer, e.g.:

Record! {
    !age: u32, // generates a label type
}

Bikeshed on the sigil of course, for example: ~age, .age, +age, #age

On the other hand, a benefit of macro_rules is that it can be in the same crate...
But perhaps we want to make poly_fn a proc macro too?