Constraints as Bounded Expressions

[mmghannam]

Build "Expressions" from variables that allow arithmetic operations, and add a range to bound them.

[matbesancon]

this will be interesting to perform in Rust. The three ways I see it are:

1. Macros that you parse to create a SCIP expression
2. Operator overloading on variables to build up expressions
3. String substitution, since you can pass SCIP a string containing the expression, the only work on the wrapper side is then to replace the expression of the variable accepted by russcip with that accepted by SCIP