[WIP] Add 'fluent interface' for visitor construction

Following an idea originally proposed by @smuething long time ago this allows to create a visitor using lambda expressions. In contrast to providing plain callbacks to a global traversal function (as in forEachNode()) this has some benefits:

• It simplifies the Visitor approach instead of creating a new traversal mechanism. Hence you still have the full flexibility of the Visitor approach.
• Instead of positional arguments you have named arguments which is far less error prone.
• You can use the existing mechanism to switch traversal types.
• You can easily extend existing visitors.
• You can chain several callbacks for a single purpose (e.g. pre(...)).

As far as I remember this design pattern is called 'fluent design'. Here we use the piping syntax which was popularized by range adaptors from C++20 (and its prototype in range-V3). Usage example:

int counter = 0;
applyToTree(tree, makeVisitor()         // create default doing nothig
  | pre([&](auto&&...) { ++counter; })  // extend by pre callback
  | leaf([&](auto&&...) { ++counter; }) // extend by leaf callback
);                                      // traverse tree and count all nodes