Overloading Rust operators for some of the `Ast` functions about z3.rs

[waywardmonkeys]

I'm interested in this ... but a couple of things have stopped me from working on it so far:

• I wasn't happy that not all operators would work, especially boolean ones.
• I'd like for this to be more type safe, instead of just having an Ast type, having Rust types for the various sorts, so that we can be sure to dispatch to the correct underlying operation.
• Perhaps having a macro that parses SMT-LIB or a simple expression language could be nice. (And this would allow all operators to work...)

But that shouldn't stop you!

A good first step might be having some additional types and then creating the operators for them?

I'm happy to see a PR! I'm also happy to have a big discussion here. :)