Optimizations around `let` variables

[mohammadfawaz]

In the future, we want do more involved things such as removing unnecessary lets (const folding, DCE, etc.). That is, those variables don't necessarily need a state program. So, someone could write:

let x = 5;
let y = storage::y + x;

the first let shouldn't need a state program while the second should.

[mohammadfawaz]

After constant folding, the above should become:

let y = storage::y + 5;