Applying map_* on list of R6 objects

[imanuelcostigan]

Issue: map_* functions no longer works when calling an active binding of an R6 class.

Reprex:

Numbers <- R6::R6Class("Numbers",
  public = list(
    x = 100
  ),
  active = list(
    x2 = function(value) {
      if (missing(value)) return(self$x * 2)
      else self$x <- value/2
    },
    rand = function() rnorm(1)
  )
)

n1 <- Numbers$new()
n1[["x2"]]
# > [1] 200
n2 <- Numbers$new()
n2[["x2"]]
# > [1] 200
nn <- list(n1, n2)
purrr::map_dbl(nn, "x2")
# > Error: `x` must be a vector (not a environment)

Session:

> session_info()
Session info ------------------------------------------------------------------------------------
 setting  value                       
 version  R version 3.3.1 (2016-06-21)
 system   x86_64, darwin15.6.0        
 ui       RStudio (0.99.1246)         
 language (EN)                        
 collate  en_AU.UTF-8                 
 tz       Australia/Sydney            
 date     2016-07-03                  

Packages ----------------------------------------------------------------------------------------
 package   * version date       source        
 devtools  * 1.12.0  2016-06-24 CRAN (R 3.3.0)
 digest      0.6.9   2016-01-08 CRAN (R 3.3.0)
 magrittr    1.5     2014-11-22 CRAN (R 3.3.0)
 memoise     1.0.0   2016-01-29 CRAN (R 3.3.0)
 purrr       0.2.2   2016-06-18 CRAN (R 3.3.0)
 R6          2.1.2   2016-01-26 CRAN (R 3.3.0)
 Rcpp        0.12.5  2016-05-14 CRAN (R 3.3.0)
 rsconnect   0.4.3   2016-05-02 CRAN (R 3.3.0)
 withr       1.0.2   2016-06-20 CRAN (R 3.3.0)

[hadley]

Related to #200: extract_impl() needs to be taught about environments