Allow `method(signature = )` to accept a `list` of length 1

[dgkf]

> library(S7)
>
> fn2 <- new_generic("fn", c("x", "y"))
> method(fn2, list(class_character, class_character)) <- function(x, y, ...) "Hello, World 2!"

So far, so good. When a generic has multiple dispatch args, this works just fine. But even after spending quite a bit of time with S7, I'm hit with this error pretty repeatedly:

> fn <- new_generic("fn", c("x"))
> method(fn, list(class_character)) <- function(x, ...) "Hello, World"
#> Error: Can't convert `signature` to a valid class. Class specification must
#> be an S7 class object, the result of `new_S3_class()`, an S4 class object,
#> or a base class, not a <list>.

In practical use this is just a bit of a nuisance. I seem to constantly be forgetting that the accepted type signature of method()<- changes based on the number of arguments. When I'm in the groove of writing up methods, I always default to writing my signature as a list() because I tend to reach for S7 specifically in the case where multiple dispatch is helpful.

It would be nice if it accepted a list as well for methods that accept a single dispatch arg for the sake of type stability.

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^{I've been on a deep dive using S7 for a package recently and am carving out time to file feedback today. Sorry for the influx of new issues - loving the package overall 🙌}