Use method<- to register methods on S3 generics for base classes

[t-kalinowski]

Currently, this raises an error:

library(S7)
an_s3_generic <- function(x) UseMethod("an_s3_generic")
method(an_s3_generic, S7::class_character) <- function(x) "foo"
#> Error: When registering methods for S3 generic an_s3_generic(), signature must be an S7 class, not a base type.

It would be nice if this worked and method<- was truly a single interface for registering any kind of method.

[hadley]

I can't remember exactly why but I think I had a good reason for doing this.

[t-kalinowski]

I think after #422, implementing this shouldn’t be too difficult, and we can quickly rediscover why it's not allowed 😆.

[lawremi]

Agreed that this would need to happen if we really see S7 replacing the legacy S3 registration mechanism.