[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, 1 insertions, 0 deletions

diff --git a/include/mbgl/style/expression/equals.hpp b/include/mbgl/style/expression/equals.hpp
index 80550bd59d..54df890a68 100644
--- a/include/mbgl/style/expression/equals.hpp
+++ b/include/mbgl/style/expression/equals.hpp
@@ -21,6 +21,7 @@ public:
     EvaluationResult evaluate(const EvaluationContext&) const override;
     std::vector<optional<Value>> possibleOutputs() const override;
 
+    std::string getOperator() const override { return negate ? "!=" : "=="; }
 private:
     std::unique_ptr<Expression> lhs;
     std::unique_ptr<Expression> rhs;