Implement Iterator over a Model

[Pat-Lafon]

One nice abstraction that the python bindings implement is to be able to iterate over a Model. One can then use the features added in #221 to get the interpretations that the Model assigns for these declarations.

If this is worth adding, the implementation comes from https://z3prover.github.io/api/html/classz3py_1_1_model_ref.html#a7890b7c9bc70cf2a26a343c22d2c8367 where they hook into Python's __len__ and __getitem__ to extract all consts and funcs from a model when using a for loop. I think the only difference here is that for Rust we need to manage a struct that does the iteration, here ModelIter(akin to https://doc.rust-lang.org/std/slice/struct.Iter.html) which contains the current index into the model and the total length.

The testcase is related to #221.

[waywardmonkeys]

Thanks!