Native construction of R6 objects

[jonathon-love]

hi,

possibly a similar motivation as #9, what do you think about native construction of the R6 objects?

i currently have the issue that the construction of R6 objects is too slow, and while i could add this capability to R6, i think w wants to keep the dependencies as light as possible (i.e. not include Rcpp)

so i could add them to RcppR6.

pros:

• faster
• doesn't actually require the user to have R6 installed (!)

cons:

• possible breakage when R6 changes (although it's pretty mature and doesn't change much).

thoughts?

jonathon