`list.count` should be using `foldr`

[KristianBalaj]

foldr seems to be more performant in Aiken when doing aggregations and count is an aggregation.

Check out the following benchmark (countl is the one currently implemented in stdlib):

fn countl(self: List<a>, predicate: fn(a) -> Bool) -> Int {
  list.foldl(
    self,
    0,
    fn(item, total) {
      if predicate(item) {
        total + 1
      } else {
        total
      }
    },
  )
}

fn countr(self: List<a>, predicate: fn(a) -> Bool) -> Int {
  list.foldr(
    self,
    0,
    fn(item, total) {
      if predicate(item) {
        total + 1
      } else {
        total
      }
    },
  )
}

And the benchmark:

test countl_1() {
  countl([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], fn(a) { a >= 2 }) == 9
}
test countr_1() {
  countr([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], fn(a) { a >= 2 }) == 9
}

Resulting in the following:

    │ PASS [mem: 47251, cpu: 18667630] countl_1
    │ PASS [mem: 46951, cpu: 18598630] countr_1