[task] elaborations on expressions

[tlambert03]

Originally posted by @funkey in #9 (review)

things we could build on top of the expressions:

• conversion of expressions into objectives (see Add conversion from Expression to Objective #13)
• support for quadratic objectives (see Support quadratic expressions to Objective #15)
• support for quadratic constraints, which are not part of the C++ wrapper, yet (see Wrap quadratic constraints #12)
• definitions of ranges (0 <= x <= 10 could be allowed)
• support for boolean expressions (x > 0 and y == 1 = z)