feature/nat_ops

[leissa]

Just sth we needed when working on #90 and is potentially useful for everyone to have.

[NeuralCoder3]

Why are nop as axioms needed?
Isn't the semantics

%core.nop.add (a,b)

the same as

.let a_i32 = %core.bitcast (I32, .Nat) a;
.let b_i32 = %core.bitcast (I32, .Nat) b;
.let c_i32 = %core.wrap.add (0,_32) (a_i32,b_i32);
%core.bitcast (.Nat, I32) c_i32

?
Even the normalization should be (nearly) the same.

[leissa]

It just makes our lives easier to directly have Nat-based operations. I think it makes sense to extend the special axiom namespace to work for all kind of entities - as I've already suggested. Then, we could define those things like this:

.lam %core.nop.add !(a: .Nat, b: .Nat) -> .Nat =
    .let a_i32 = %core.bitcast (I32, .Nat) a;
    .let b_i32 = %core.bitcast (I32, .Nat) b;
    .let c_i32 = %core.wrap.add (0,_32) (a_i32,b_i32);
    %core.bitcast (.Nat, I32)