Improve simplification of arithmetic

[emilaxelsson]

The current implementation is quite incomplete and it only deals with merging literals, so it can't cancel out variables; e.g.

(x+x)-x  ===>  x

It would be better to optimize a whole arithmetic expression at once. Gather all variables in one list, all literals in one list and all non-arithmetic sub-terms in one list. Then make a new optimized expression by combining the three lists.

However, doing this compositionally will probably lead to a lot of re-traversals of the same sub-terms, so the optimization framework will probably have to be modified so that arithmetic optimization only happens at feasible places (i.e. arithmetic sub-terms whose parents are not arithmetic expressions).

We should also think about how to deal with examples like this one (reported by Gergely):

> icompile (\(n::Data Word8) -> (n-2)+1)
...
void test(uint8_t v0, uint8_t * out)
{
    (* out) = (v0 + 255);
}

The result is correct, but not ideal from a readability point of view.