[NFC] Experiment with C++20 coroutines for expression traversal

[tlively]

Add a gtest file that is only included in the build when using C++20 or greater that defines a generator utility in terms of C++20 coroutines and then uses that to define generators for traversing Expressions. Unlike our current traversal utilities, these generators leave control flow up to their callers, making it trivial to do things like stop the traversal early or run multiple traversals concurrently, for example to compare two expression trees.

[tlively]

• #5447 👈
• main

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