[core] Implement Expression::serialize()

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`

1 files changed, 2 insertions, 0 deletions

diff --git a/include/mbgl/style/expression/step.hpp b/include/mbgl/style/expression/step.hpp
index 6bf42e20f1..2f9524a53c 100644
--- a/include/mbgl/style/expression/step.hpp
+++ b/include/mbgl/style/expression/step.hpp
@@ -38,6 +38,8 @@ public:
 
     static ParseResult parse(const mbgl::style::conversion::Convertible& value, ParsingContext& ctx);
 
+    mbgl::Value serialize() const override;
+    std::string getOperator() const override { return "step"; }
 private:
     const std::unique_ptr<Expression> input;
     const std::map<double, std::unique_ptr<Expression>> stops;