Returns an integer of the size of the object.
For primitives (booleans, characters, numbers) returns 0.
For Strings and java vectors, returns the length.
For Records, returns a non-negative number that's the lowest integer
for which there's no key of that integer.
Usually when you've got a record and some integer named fields,
you'll have consecutive integers up from zero like so: 0, 1, 2, 3
in which case length would return 4.
But if you had integer fields of: 0, 1, 3, 4,
length would return 2.