Fix IRAnything

[schani]

IRAnything currently serves two purposes in the IR:

1. In inference, to signify a type we know nothing about. The only way for it to come about is as the element type of an empty array.

2. In generation, for any type, which we translate to object in C#.

We make no explicit distinction between the two in the code, i.e. they are one and the same.

That mostly works for now, but we will run into trouble if our inference algorithm ever wants to distinguish between the two. In addition to that, JSON Schema does have a representation for "any type", but our unification algorithm does not treat IRAnything as "any type", so we can't use and expect to get correct results. For example, if we treat "any type" as IRAnything, we would unify an array that contains one "any type" and one integer as an array containing integers. The correct unification is an array containing "any type".

If we had IRNoInformation and IRAny, unification would look like this:

unify IRNoInformation x = x
unify IRAny x = IRAny

Of course, to preserve more type information we could still do

unify IRAny x = IRUnion [IRAny, x]