implement swank:interactive-eval and swank:eval-and-grab-output

This allows C-c : and C-u C-c : to work, modulo the printing of an
unnecessary [1].  The evaluation semantics of R are not what a Lisper might
expect; printToString(eval(parse(...))) does not necessarily perform the
evaluation before altering the output stream with sink() -- so capturing
all sorts of incidental output if something goes wrong.

This commit brought to you using C-c C-c.