Refactor derives

[Diggsey]

This should make it much easier to extend these derives to support enums.

The only change to behaviour should be that LabelledGeneric can now be derived on unit structs.

Changes to the generated code:

• Renamed '_frunk_labelled_generic_ref_ to _frunk_ref_ because the new code is no longer specific to labelled generics. It would be easy to extend the "ref" implementations to support plain Generic if desired.

• Instead of extracting the value from Fields using property accessor syntax (field.value) it is destructed directly from the HList, using Field { value: <x>, .. }. This has the advantage of allowing the construction and destructuring of the user's type to use identical syntax.

Changes to naming conventions:

• There are three main "places" where generated code is used: types, expressions and patterns. This is complicated by the fact that much syntax is valid as both an expression and a pattern. The convention I used is for build_xxx_constr methods to be usable in both expression and pattern locations, whilst build_xxx_type, build_xxx_expr and build_xxx_pat methods are self explanatory.

• For normal/ref/mutable ref variations, always use the suffixes /_ref/_mut.

There are several features that could be added as follow-ups to this PR:

• Addition of IntoGeneric trait and implementation of Generic for references to types deriving Generic.
• Support for deriving Generic and LabelledGeneric on enums.

Are these features you would want to add?

[lloydmeta]

Thanks so much for doing this. Indeed, this area of the code has been needing some additional love for a while, exactly for the reasons you cited (better readability, extendability)

There are several features that could be added as follow-ups to this PR:

• Addition of IntoGeneric trait and implementation of Generic for references to types deriving Generic.
• Support for deriving Generic and LabelledGeneric on enums.

Are these features you would want to add?

Most definitely, those would be amazing to have. If I was a prioritising man, I would say the deriving LabelledGeneric for enums would probably be on the top of that list :)

Also, sorry for the delayed response; I'm currently on vacation and will take a closer look when I'm back and settled in. Looking awesome thus far !

[Diggsey]

I've added a rustfmt commit.