Implement recursive reduction using Zia

[Charles-Johnson]

Now that #20 has been resolved, it should be possible to

let ((_y_ exists_such_that) (_x_ -> _y_) and _y_ -> _z_) => _x_ -> _z_

which would remove the need for ContextSearch::recursively_reduce.

This will allow the interpreter to reason more generally.

One could go further and generalise the transitive properties shared with comparison and reduction with the statements

let (_o_ is transitive) => ((_y_ exists_such_that) (_x_ _o_ _y_) and _y_ _o_ _z_) => _x_ _o_ _z_

and

let -> is transitive

and

let > is transitive

This will be a good test for the reasoning of the interpreter

[Charles-Johnson]

I should open a PR testing this