Implement $ and $<- methods

[hadley]

Fixes #198

[DavisVaughan]

Somewhat interesting to note that if:

# rather than this, where the integer vector is the core data object
new_class("foo", parent = "integer")

# we instead used this as the implementation
# (where new_class("foo", parent = "integer") would basically do this instead)
new_class("foo", properties = list(data = "integer"))

Then we could drop @ altogether and use $ to access the properties, one of which would be $data.

I know you mentioned that we probably want a way to directly build on base atomic objects, so it probably is too late, but it is interesting to think about how @ could be removed (that would probably make autocomplete work on properties out of the box too, probably through .DollarNames)

[DavisVaughan]

I guess you still have to think about whether or not you want this with lists though

# if this:
new_class("foo", parent = "list")

# was basically this:
foo <- new_class("foo", properties = list(data = "list")

obj <- foo(list(x = 1))

# then you'd have to do this:
obj$data$x

I'm not sure I mind this though, since the argument name is foo(.data=). And it isn't like we were able to do obj <- foo(x = 1), so I could be convinced that I should expect to have to go through $data to get to the list elements

That would also remove this ambiguity about when you can use $ on an R7 object

[hadley]

I think the problem is then that is.list() would be TRUE for R7 objects which seems likely to lead to a bunch of unintended downstream consequences.

[DavisVaughan]

I was still thinking the core object would be that weird S4 gremlin thing that is returned by R7_object(), and we'd still have all the properties as attributes on that

[DavisVaughan]

Same question about expressions