There are a lot of nodes that cannot be removed or converted to maybes, such as
most of the Op tokens. It would be a bit of a lie to codegen leave_* methods
that allow these nodes to be converted, only to throw a runtime error later. So,
upgrade the codegen to allow us to see whether certain nodes are used in conjunction
with a MaybeSentinel/None, or inside a Sequence, to inform ourselves as to when to
allow MaybeSentinel or RemovalSentinel.
We want to make sure that the generated function stubs stay in sync with
the node definitions. So, make a unit test that fails if codegen generates
a different file than the existing file, so that somebody modifying code
knows they need to re-run codegen.
