Use a single template function for each of {to,from}ExpressionValue, always dispatching to ValueConverter.
Remove unused ValueConverter::ExpressionType typedefs.
Issue #10714
- Each expression stores its operator as a string, and default serialization is [operator, serialize(child1), ...]
- Custom implementations of `serialize` for Expression types that don't follow the pattern
- expression::Value -> mbgl::Value converter
- node_expression bindings to expose `serialize`
