I'm going to extract generic alias substitution functions, and these AST types
will be accessed there. Revset parsing will also be migrated to the generic
functions.
This will help extract common FunctionCallNode<'i, T> type. We don't need
freedom of arbitrary error type choices, but implementing From<_> is the
easiest option I can think of. Another option is to constrain error type by
the expression type T through "T::ParseError: ArgumentsParseError" or
something, but it seemed a bit weird that we have to use trait just for that.
