Support for sending custom expressions to the solver

[UnsignedByte]

Hi,

easy_smt currently provides nice access to creating custom atoms and using them in custom expressions, but has no way to actually send these to the solver for the user (as solver.send is private). This is useful for example with optimization in z3 which provides special (minimize expr) and (maximize expr) expressions that need to be sent to the solver.

Such expressions can be made using the following

let minimize = self.sol.atom("minimize");
let expr = self.sol.list(vec![minimize, self.minimize_expr.clone()]);

self.sol.send(expr); // missing

but they cannot yet be sent to the solver. This should be a relatively simple patch to include a wrapper for solver.send.

[fitzgen]

This is definitely something we should support. Would you be open to whipping up a PR?