the only case I can think of where this is necessary is if you want null as a default
in ColExpr.map (if you pass None, it recognizes it as a missing argument and uses the
expression map was called on as a default.

Explicit annotations are much more readable and these will likely not change in
the future.

Remove the builtin_verb decorator. We still need to decide whether materialize
should fail on a dataframe backend or do nothing.

includes self-join detection and seeing whether a column is actually in
the table

alias() currently breaks table references which is necessary for self-joins
to work. We could add another verb that doesn't do that.

subqueries were falsely generated for window functions nested inside ewise
functions, even though they are not required there.