Allow class-level members

[iqis]

Some OO designs may make use of a class-level member.
For example, incrementing a serial number:

Employee <- R6::R6Class("Employee", 
                        class_member = list(count = 0), 
                        public = list(
                            initialize = function(name){
                                self$name <- name
                                self$class$class_member$count <- 
                                    self$id <- 
                                    self$class$count + 1
                            }, 
                            id = NULL
                            name = NULL
                        ))
john <- Employee$new("John")
mary <- Employee$new("Mary")

john$id
# [1] 1
mary$id
# [1] 2

Depends on #225.

[wch]

This sounds similar to having Java-style static members: #222 (comment)

[iqis]

Thanks, I think this is an exact duplicate... Wasn't using static when searching for existing issues.