In order to test the conversion from function to expressions using the
legacy tests in the expression test suite, parse function objects into
expressions in NodeExpression::Parse.
v2: Coding style fixes.
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`
