Add (or document if already exist) ability to derive custom typeclasses via source plugins

Problem

Suppose, I have some custom typeclass Foo defined in some library foo:

class Foo a where
    ... some methods ...

I would like to be able to derive instances of this typeclass for any possible data type using deriving clause just like GHC already does for typeclasses Eq, Ord, Show, Read, Enum, etc.:

data Bar = Bar | Baz
    deriving (Eq, Ord, Foo)

There're already two possible ways to derive instances of custom typeclasses:

1. anyclass deriving strategy (usually involves Generic)
2. -XTemplateHaskell solution.

But I would like to have source-plugin-based solution for this problem so I can just add -fplugin=Foo.Plugin and enjoy deriving capabilities.

Advantage over existing approaches

Solution with -XTemplateHaskell is not that pleasant to write and easy to maintain (you need to use libraries like http://hackage.haskell.org/package/th-abstraction to support multiple GHC versions),involves scoping restriction and is syntactically uglier. Compare:

{-# LANGUAGE TemplateHaskell #-}

data Bar = Bar | Baz
    deriving (Eq, Ord)

deriveFoo ''Bar

Solution with something like Generic introduces performance overhead (required for to/from generic representation conversion). This might not be significant for something like parsing CLI arguments but it's more important if you want to have efficient binary serialisation.

Also, it's known that deriving typeclasses is a relatively slow compilation process (https://github.com/tfausak/tfausak.github.io/issues/127) so there's slight chance that deriving typeclass manually can be slightly faster than deriving Generic + MyClass. Especially when maintainers of plugins can experiment with some caching strategies for deriving typeclasses.

Trac metadata

Trac field	Value
Version	8.6.1-beta1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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