Allow multiple column computations in data masking slides

[brookslogan]

Currently, only a single data masking expression can be fed into epi[x]_slide; e.g., trying to compute 7davs of two signals simultaneously requires using a function/formula f or as_list_col=FALSE. (Plus, the evaluation environment(s) for the data masking expression might not quite be correct, and as_list_col=FALSE on a single data masking input doesn't match dplyr patterns, plus adds a name prefix that is very likely unwanted for the tidyeval case and maybe in general.) Allow multiple data masking expressions to be provided, and make sure to pass appropriate environments. Maybe give some extra thought to make sure dplyr::cur_* functions work properly, although note these caveats.

[brookslogan]

• Check, e.g., advanced.Rmd to remove potential references to epix_slide outputting one row "for element/row output from calls to f".

[brookslogan]

• Look at ?as_data_mask, specifically the mention of lst, which probably applies here as well

[dshemetov]

This is done as of #477 and possibly earlier @brookslogan ?

[brookslogan]

#477 sounds right for the multiple column computations; row requirements were relaxed earlier and it looks like the docs were updated already.