Rework vec_case_match() on vctrs::vec_case_when()

[DavisVaughan]

This effectively freezes case_match()'s implementation in preparation for it to be superseded by recode_values().

This removes the final usage of dplyr:::vec_case_when() outside of case_when() itself, which will come next, allowing us to delete that dplyr helper entirely.

[DavisVaughan]

Unlike dplyr:::vec_case_when(), vctrs::vec_case_when() is well defined when you don't give it any inputs. It's useful for programmatic reasons.

But case_when() and case_match() don't support this since we don't want to expose the unspecified type to normal users.

So I've recreated the dplyr:::vec_case_when() error here.

library(vctrs)

vec_case_when(
  cases = list(),
  values = list()
)
#> <unspecified> [0]

vec_case_when(
  cases = list(),
  values = list(),
  size = 2
)
#> <unspecified> [2]

vec_case_when(
  cases = list(),
  values = list(),
  default = 1L
)
#> integer(0)

vec_case_when(
  cases = list(),
  values = list(),
  ptype = integer()
)
#> integer(0)

[DavisVaughan]

Unlike with if_else(), we are going to have to accept this new error message where you don't have the label.

Essentially we need the common type of list_of_rhs and default, which we do at C level via

ptype <- vec_ptype_common(!!!list_of_rhs)
ptype <- vec_ptype2(ptype, default, y_arg = default_arg)

https://github.com/r-lib/vctrs/blob/c46809139a63699bde2f1b4b58286553c2f2075e/src/list-combine.c#L1769-L1823

There is no easy way to smush default (and default_arg) into the list_of_rhs at the C level compute the common ptype all in one shot. This computation actually takes place in list_combine(), where the list_of_rhs could be a very large list of objects we want to combine, so allocating a list of size + 1 just to insert default isn't worth it.

[DavisVaughan]

I think this is actually more correct. We are recycling each value here to the size of x.