Let assignment with an integer divide by 0 panics

If you run these commands:

let val = 1
let val //= 0

ion panics with the error: thread 'main' panicked at 'attempt to divide by zero', src/lib/shell/assignments.rs:616:60 note: Run with RUST_BACKTRACE=1 for a backtrace.

Solution: Add a method for checking for divide by zero and either return NaN like normal division does, or add an error which can result from such an operation (Especially since integer divide should return an integer and not a float which does support NaN)