Further Expression streamlining

[terrorfisch]

PR #750 introduces a new expression subpackage that defines an expression protocol. To allow merging some of the originial TODOs are postponed and tracked here.

The motivation is a more performant expression evaluation.

Alternative: Another solution that allows slower evaluation is by making the ForLoopPT instantiation more performant via a good ProgramBuilder implementation #782.

• ExpressionVector wrapper
• Triage which expression internals uses are fine and which are not
• Automate expression protocol testing