Move all expression evaluation from host into Executor

[mversic]

Executor should be the only entity that knows how to evaluate expressions. Host should have no reference to expressions, expressions are strictly an Executor concept and can be changed at runtime by updating Executor.

As a consequence evaluation step will now happen entirely before the validation step.
Currently, Validator/Executor traverses syntax tree and evaluates any expression encountered along the way. That is not good because it results in repeated evaluation of expressions in both executor and on the host. Executor should first evaluate every expression (such as InstructionExpr) into a concrete instruction (such as InstructionBox) and only then call visit_instruction.

This means that visit_* method signatures should be modified to operate on evaluated instructions