fix #20997 calling system.card[T](x: set[T]) with T of int8 or uint8 … (#21010)

* fix #20997 calling system.card[T](x: set[T]) with T of int8 or uint8 uses mismatched C array sizes
* fullfil set variant

tests/sets/t20997.nim

@@ -0,0 +1,18 @@
+discard """
+  joinable: false
+"""
+
+{.passC: "-flto".}
+{.passL: "-flto".}
+
+template f(n: int) = discard card(default(set[range[0 .. (1 shl n) - 1]]))
+f( 7)
+f( 8)
+f( 9)
+f(10)
+f(11)
+f(12)
+f(13)
+f(14)
+f(15)
+f(16)