Separate notions of expression constness and side-effectfulness

[SaladDais]

Right now a node returning a null ->get_constant_value() can mean that either a constant value for the node can't be determined, or that something in the expression might have side-effects.

Basically, we should be able to tell that a if ((non_const = 3) + 1) branch will always be taken because the expression will always result in 4, but that the expression can't be folded due to the side-effects of the assignment.

We should also be able to prune a declaration like string foo = non_const + "bar"; because even though we can't determine a constant value for the expression, we know that it doesn't have any side-effects.