Consider handling rrules for kwargs differently?

The lowering for a call like:

f(args...; kwargs...)

is

Core.kwfunc(f)(kwargs, args...)

By the usual rules of rrule invocation, the rrule for this call is:

rrule(Core.kwfunc(f), kwargs, args...)

However, this misses the rrules we define here, which instead expect:

Core.kwfunc(rrule)(kwargs, f, args...)

This is not forces every AD system to have special case rules for detecting kwcalls of this sort, which is non-trivial.
An alternative would be to make the rrule(Core.kwfunc(f), kwargs, args...) call primitive, though of course that would require changing the user-facing way to write rrules. This could then be fixed with a macro, but of course ChainRules currently does not use a macro for regular rrule/frule definitions... Neither alternative is particularly pleasant, but I figured I'd open this for discussion.

Start by tracing the lowering of calls shaped like f(args...; kwargs...) and compare it with the existing Core.kwfunc rrule definitions described in the issue. Review how regular rrule and frule definitions are represented, then determine which API behavior or lowering rule should change; the issue does not define a settled implementation or completion test.