Make r_process and r_session not cloneable

[karawoo]

Fixes #284

The dev version of R6 complains about a few other issues in callr -- would you like to make these finalize() methods private too? I can update the PR if so.

R6Class r_process: finalize() method is public, but it should be private as of R6 2.4.0. This code will continue to work, but in a future version of R6, finalize() will be required to be private.
R6Class r_session: finalize() method is public, but it should be private as of R6 2.4.0. This code will continue to work, but in a future version of R6, finalize() will be required to be private.
R6Class rcmd_process: finalize() method is public, but it should be private as of R6 2.4.0. This code will continue to work, but in a future version of R6, finalize() will be required to be private.
R6Class rscript_process: finalize() method is public, but it should be private as of R6 2.4.0. This code will continue to work, but in a future version of R6, finalize() will be required to be private.

[gaborcsardi]

Thank you!

[gaborcsardi]

would you like to make these finalize() methods private too? I can update the PR if so.

Thanks, I'll fix these in a sec.