Simplify ToolVisitor

• have visit accept any 'invocable' with the right signature
• make inheritance from IVisitor a (private) implementation detail
• adapt IVisitor implementations by removing their inheritance, and changing 'visit(...)' into 'operator()(...)'.
• remove the now redundant constVisitor and visitor methods to explicitly convert eg. a lambda into an IVisitor -- just pass the original argument directly to visit instead, and it will take care of this
• remove redundant 'virtual' when 'override' is already specified
• prefer std::deque over std::list
• prefer STL algorithms over explicit loops
• prefer plain values over rvalue references as arguments in a few places

Closes #142 (closed)