Clean up xmap classes

[cynthiahqy]

Clean up the spaghetti code that implements the xmap_tbl class in preparation for an xmap_list class in #98

• 92

• 94

Will not implement:

• 89

[cynthiahqy]

:fire: for reference xmap_tbl subclass had too many classes:

#'
.get_xmap_subclass_attr <- function(subclass = c("xmap_df", "xmap_tbl")){
  subclass <- rlang::arg_match(subclass)

  class_attr <- switch(subclass,
         xmap_df = c("xmap_df", "xmap", "data.frame"),
         xmap_tbl = c("xmap_tbl", "xmap_df", "xmap", "tbl_df", "tbl", "data.frame"),
         stop("Unknown xmap subclass"))

  return(class_attr)
}