Framework for copy-on-write visitors (WIP)

[ChrisDodd]

This is still very much a work in progress (doesn't even build at the moment due to order-of-declaration issues), but is complete enough to show my thinking.

The idea here is address #4418 (excessive cloning in Transform/Modifier visitors) by doing lazy clones -- nodes are only cloned when they are modified (true copy on write), by giving the preorder/postorder methods "smart" pointers that manage the cloning when fields are actually modified. This uses the ir generator to make special COWref reference objects for each IR class that understand the fields in the IR class and can be assigned to as if they were instances of the class, managing the cloning on demand.

Since we've been saying we're going to update to requiring C++20, this uses C++20 concepts in a number of places to simplify the overloading.

[asl]

@ChrisDodd Great! How does it handle children visiting? E.g. when a child is changed, how do we clone the parent? Especially when e.g. inside Vector or IndexedVector?