Move most instructions to expressions

[samsa1]

Currently many things that are considered instructions could be put in the instructions such as returns.

Currently the user cannot write :

fn my_fun(b : bool) -> i32 {
    if b {
       return 1
    };
    expr
}

This modification will impact the type inferencer!

[samsa1]

This code is now allowed by #15 however the following code is not accepted by typing :

fn my_fun() -> i32 {
     return 1;
}

The reason of this is because expr; has type unit instead of i32.

A divergence type should also be added to the compiler.