Puts the length of the top item on the stack back to the top of the stack
LENGTH pops a top item off the stack and pushes its length back to the
top of the stack.
PumpkinDB> "Hello" LENGTH 5
Allocates for the result of the item size calculation
EmptyStack error if there are no items on the stack
works : "123" LENGTH 3 EQUAL?. empty_stack : [LENGTH] TRY UNWRAP 0x04 EQUAL?.