use language objects when processing code

[chlebowa]

In lieu of @pawelru following discussion in #1015

Rewrite functions to reconstruct code in .datasets_to_data (R/module_nested_tabs.R) and their dependencies so that they operate on language rather than character.

[gogonzo]

We use language object when processing code when it is needed. parse(text = <char code>) is super quick and reliable. We store code as character to preserve formatting and comments asis