Implement r_to_py for external pointer

[Marlin-Na]

Fixes #660

[Marlin-Na]

An example can be:

library(xptr)
library(reticulate)
xptr <- xptr::new_xptr("1122")
xptr
# <pointer: 0x462>
r_to_py(xptr)
# <capsule object NULL at 0x11938bb70>

[kevinushey]

Thanks for the pull request!

We require a Contributor License Agreement before we can accept pull requests on RStudio repositories. Would you be willing to fill one out, and send it to the e-mail indicated in the form? (Please also reference me and this PR in your e-mail.)

[Marlin-Na]

Hi @kevinushey , I have emailed the agreement.

Best,
Jialin

[kevinushey]

Thank you!