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Some are easier than others.

I'm starting to evolve a (still ad-hoc) naming convention for the
extracted functions in order to provide some cues for how common you can
expect the thing to be.  Scalars have a lot more commonizable stuff
than non-scalars, for example.

These are getting increasingly straightforward as most of the hard
problems have been ironed out already when getting the other recursives
(maps and structs) sorted.

Now, we just need to stamp out the rest of the scalars, and I think
this codegen stuff is at least early-alpha level usable!

Also in this commit: added some new error types and fixed up the
basicnode.List implementation to use it, removing some todos there.