Add wrapping type to allow opt-out from safety checks

### What is wrong?

Now that #153 landed Fe performs various safety checks for arithmetic operations by default. This also means that currently one can not write code that deliberately relies on over or underflow.

Long term it would be good to allow people to opt-out from these safety guarantees as long as it is very explicit.

### How can it be fixed

Preferably do what Rust is doing with the [Wrapping](https://doc.rust-lang.org/std/num/struct.Wrapping.html) type.

Example:

```
def add_with_overflow(val1: u256, val2: u256) -> u256:
return wrapping(val1) + wrapping(val2)
```

Start by reviewing the arithmetic safety checks introduced by #153 and the linked Rust Wrapping type documentation. The issue provides no Fe files or tests to begin with; done would mean an explicit wrapping type or equivalent syntax that allows deliberate overflow and underflow.

Written by the indexing model from the issue text.