Modularize handling of special functions and NSE in recording

[epatters]

Currently special functions and non-standard evaluation (NSE) are handled (to the extent they're handled all) by ad hoc conditional logic in the main recording function.

Due to the design of R, a certain amount of ad hoc logic will always be necessary, but it should be modularized and de-coupled from the generic recording logic. This will make it easier to, e.g., write handlers for various forms of NSE.