Add implementations for std::ops traits that work on references

[krtab]

For the Add trait, std has:

• impl Add<f64> for f64
• impl<'a> Add<f64> for &'a f64
• impl<'_> Add<&'_ f64> for f64
• impl<'_, '_> Add<&'_ f64> for &'_ f64

This PR adds the last three for ConstrainedFloat, for Add, Mul, Sub, Div, Rem.

It also add similar implementations for {Add,Mul,Sub,Div,Rem}Assign and Neg.