Don't use inheritance for expression classes

[vitaut]

This shouldn't be allowed, because it undermines type safety:

  auto n = factory_.MakeNumericConstant(42);
  auto u = factory_.MakeUnary(expr::ABS, n);
  mp::Expr &e = n;
  e = u;

So make expression classes independent types instantiated from a common template instead of using inheritance. This is similar to smart pointers and effectively expression classes are like smart pointers but with a nice API.

[vitaut]

Fixed for new expression types as of 02ca9fa. Need to do the same for ASL expressions.

[vitaut]

Complete in 449ba37.