Add similar dput function to node sets

[cimentadaj]

Issue Description and Expected Result

For reprex examples it might be useful to have an equivalent dput for nodes/nodesets.

A 2 nodeset example from dput which crashes when saved.

x <- structure(list(structure(list(node = <pointer: 0x111b131d0>, 
    doc = <pointer: 0x101b85230>), .Names = c("node", "doc"), class = "xml_node"), 
    structure(list(node = <pointer: 0x111b13330>, doc = <pointer: 0x101b85230>), .Names = c("node", 
    "doc"), class = "xml_node")), class = "xml_nodeset")

Session info -------------------------------------------------------------------------------------------------------------------------
 setting  value                       
 version  R version 3.4.3 (2017-11-30)
 system   x86_64, darwin15.6.0        
 ui       RStudio (1.1.383)           
 language (EN)                        
 collate  en_US.UTF-8                 
 tz       Europe/Madrid               
 date     2018-02-12                  

Packages -----------------------------------------------------------------------------------------------------------------------------
 package   * version date       source                         
 base      * 3.4.3   2017-12-07 local                          
 compiler    3.4.3   2017-12-07 local                          
 datasets  * 3.4.3   2017-12-07 local                          
 devtools    1.13.4  2017-11-09 CRAN (R 3.4.2)                 
 digest      0.6.15  2018-01-28 cran (@0.6.15)                 
 graphics  * 3.4.3   2017-12-07 local                          
 grDevices * 3.4.3   2017-12-07 local                          
 memoise     1.1.0   2018-02-08 Github (hadley/memoise@611cfad)
 methods   * 3.4.3   2017-12-07 local                          
 stats     * 3.4.3   2017-12-07 local                          
 tools       3.4.3   2017-12-07 local                          
 utils     * 3.4.3   2017-12-07 local                          
 withr       2.1.1   2017-12-19 cran (@2.1.1) 

[jimhester]

It isn't actually possible to define methods for dput() or dump(), and XPtrs output as you show, so there unfortunately isn't actually much we can do about this.