updated formatting in print.R6 (fixes #173)

[jameslamb]

@wch On #171 , you recommended I look at #173 .

Please consider this PR that I think addresses it.

An example:

thing <- R6::R6Class(
    classname = "thing",
    public = list(
        some_func = function(x = TRUE){NULL}
        , x = 5
    )
)
print(thing)

[jameslamb]

@wch Friendly reminder about this small PR

[wch]

I think it makes more sense to insert the end result here, instead of marking it as "function" and replacing later. This is because it keeps things simpler, and because it avoids the possibility of an error if a member is the string "function".

Something like:

     if (is.function(obj)) {
       txt <- deparse(args(obj), width.cutoff = 60L)
       txt <- txt[-length(txt)]  # Drop last line, which is "NULL"
       paste(txt, collapse = "\n")
     }

Note that the value I used for width is a bit arbitrary.

Ideally, it would wrap so that the total width (including indentation that is added later) is less than or equal to getOption('width'), but I think the way deparse uses width.cutoff is that it wraps after that number of characters.

[jameslamb]

I am going to close this. Someone else can pick up the issue.