More class introspection helpers

I'm working on S7 support for roxygen2, and it'd be useful to expose more of the internal helpers for turning class names into strings. For example, if I'm documenting this method: ```R #' A title method(generic, list(x, y)) <- function(x, y) { ... } ``` I'd like to be able to more easily generate this comment in the usage: ```Rd \usage{ # S7 method for class_x, class_y generic(x, y) } ```