Expand as_slide_computation to perform f function validation and convert tidy computations into functions

[nmdefries]

• [easy] Have as_slide_computation handle function validation via assert_sufficient_f_args as well as converting tidy computations into functions.

  Currently, as_slide_computation is only used to handle user-provided formulas, but it supports functions too -- functions are returned as-is at the moment. It's easy to move the call to assert_sufficient_f_args into as_slide_computation from epi[x]_slide.

• [moderate] Have as_slide_computation convert tidy computations into functions. Bonus that this lets us deduplicate the function/formula and tidy expr logical branches in epi[x]_slide.

  There are a couple of difficulties here. epi_slide's and epix_slide's tidy expr-to-function construction are a little different, e.g. epi_slide's needs access to some objects from within epi_slide (before). The tidy expr logical branches in epi[x]_slide also need to pass quo around, to group_modify, e.g., so we might need to check if quo is included in ... to trigger certain behavior.