Add basic WLP calculation

The WLP of a subset of statements in the language can now be calculated.
This needs to be extended, but it gives a basic idea of what we can do.

I made new types for the logical expressions (and the mathematical
expression in them) rather than reusing the datatype within the
language. I feel that there's a benefit to separating these, showing
when we're talking about the programs and when we're talking about the
wlp and its calculation and the like. However, this does increase the
amount of types we use. I think the benefit outweighs the cost of having
extra types, but I also feel like we should discuss this :)