Possibly use a vector of nodes to represent rope concat nodes

[catamorphism]

As per FIXME in std::rope.

[catamorphism]

Also: try to refactor code (probably by using classes) to avoid pattern-matching in things like byte_len, and use memcpy in serialize_node. I have a branch that tries to do these things, but I got test failures and probably did something subtly and horribly wrong that I don't want to debug at the moment.

[emberian]

Will be closed by #7629