Move public rule functions out of rule namespace

This way, there's only one public namespace: tree_sitter

src/compiler/rule.h

@@ -0,0 +1,43 @@
+#ifndef COMPILER_RULES_RULE_H_
+#define COMPILER_RULES_RULE_H_
+
+#include <string>
+#include <memory>
+
+namespace tree_sitter {
+
+namespace rules {
+class Visitor;
+}  // namespace rules
+
+class Rule;
+typedef std::shared_ptr<Rule> rule_ptr;
+
+class Rule {
+ public:
+  virtual bool operator==(const Rule &other) const = 0;
+  bool operator!=(const Rule &other) const;
+  virtual size_t hash_code() const = 0;
+  virtual rule_ptr copy() const = 0;
+  virtual std::string to_string() const = 0;
+  virtual void accept(rules::Visitor *visitor) const = 0;
+  virtual ~Rule();
+};
+
+std::ostream &operator<<(std::ostream &stream, const Rule &rule);
+std::ostream &operator<<(std::ostream &stream, const rule_ptr &rule);
+
+}  // namespace tree_sitter
+
+namespace std {
+
+template <>
+struct hash<tree_sitter::rule_ptr> {
+  size_t operator()(const tree_sitter::rule_ptr &rule) const {
+    return typeid(*rule).hash_code() ^ rule->hash_code();
+  }
+};
+
+}  // namespace std
+
+#endif  // COMPILER_RULES_RULE_H_