Add `Active bindings` section to default `print` method for `R6` class instances

[mihaiconstantin]

Consider the following class (i.e., with intentionally confusing member names):

SomeClass <- R6::R6Class("SomeClass",
    private = list(
        .a = NULL,
        .c = NULL
    ),

    public = list(
        e = NULL,

        initialize = function() {
            # ...
        },

        b = function() {
            # ...
        },

        f = function() {
            # ...
        }
    ),

    active = list(
        g = function() {
            # ...
        },

        d = function() {
            # ...
        }
    )
)

Printing the generator yields:

<SomeClass> object generator
  Public:
    e: NULL
    initialize: function () 
    b: function () 
    f: function () 
    clone: function (deep = FALSE) 
  Active bindings:
    g: function () 
    d: function () 
  Private:
    .a: NULL
    .c: NULL
  Parent env: <environment: R_GlobalEnv>
  Locked objects: TRUE
  Locked class: FALSE
  Portable: TRUE

Similarly, printing an instance yields:

<SomeClass>
  Public:
    b: function () 
    clone: function (deep = FALSE) 
    d: active binding
    e: NULL
    f: function () 
    g: active binding
    initialize: function () 
  Private:
    .a: NULL
    .c: NULL

It appears that the members under each access modifier are sorted alphabetically. Would it make sense for the default print method for instances to include an Active bindings section, similar to the object generator? I think this could improve clarity, especially for more complex classes and inheritance chains.