Determines the number of children in a container
This includes variants, maybes, arrays, tuples and dictionary
entries. It is an error to call this function on any other type of
For variants, the return value is always 1. For values with maybe types, it is always zero or one. For arrays, it is the length of the array. For tuples it is the number of tuple items (which depends only on the type). For dictionary entries, it is always 2
This function is O(1).