Add fmt field tags to core:container/small_array.Small_Array.

This enables fmt (and other packages that use it, like log) to format
Small_Arrays with only the used portion of the array.

core/container/small_array/small_array.odin

@@ -21,7 +21,7 @@ Example:
 	}
 */
 Small_Array :: struct($N: int, $T: typeid) where N >= 0 {
-	data: [N]T,
+	data: [N]T `fmt:",len"`,
 	len:  int,
 }