Move public rule functions out of rule namespace

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

src/compiler/rule.cc

@@ -0,0 +1,27 @@
+#include "compiler/rule.h"
+#include <set>
+
+namespace tree_sitter {
+
+using std::ostream;
+using std::string;
+
+bool Rule::operator!=(const Rule &other) const {
+  return !this->operator==(other);
+}
+
+ostream &operator<<(ostream &stream, const Rule &rule) {
+  return stream << rule.to_string();
+}
+
+ostream &operator<<(ostream &stream, const rule_ptr &rule) {
+  if (rule.get())
+    stream << *rule;
+  else
+    stream << string("(null-rule)");
+  return stream;
+}
+
+Rule::~Rule() {}
+
+}  // namespace tree_sitter