feat!: Remove Function and Optimizer from the prelude

Having both Solver and Optimizer traits in the prelude, when a type
implements both, the compiler can't distinguish between them for a call
to `next` method. Given that function optimization is not the main goal
of gomez, but just a by-product, as a temporary solution
function-related traits are removed from the prelude.

However, the design of the traits should be somehow improved so that the
clash is not there at all.

src/core/function.rs

@@ -18,6 +18,7 @@ use super::{
 /// two required methods: [`apply`](Function::apply) and [`dim`](Problem::dim).
 ///
 /// ```rust
+/// use gomez::core::Function;
 /// use gomez::nalgebra as na;
 /// use gomez::prelude::*;
 /// use na::{Dim, DimName, IsContiguous};

src/lib.rs

@@ -261,10 +261,7 @@ pub use nalgebra;
 /// Gomez prelude.
 pub mod prelude {
     pub use crate::{
-        core::{
-            Domain, Function, Optimizer, Problem, ProblemError, Solver, System, Variable,
-            VariableBuilder,
-        },
+        core::{Domain, Problem, ProblemError, Solver, System, Variable, VariableBuilder},
         var,
     };
 }