Put rule classes in their own files

src/rules/symbol.cpp

@@ -0,0 +1,25 @@
+#include "symbol.h"
+#include "transition_map.h"
+
+namespace tree_sitter  {
+    namespace rules {
+        Symbol::Symbol(int id) : id(id) {};
+
+        TransitionMap<Rule> Symbol::transitions() const {
+            return TransitionMap<Rule>({ copy() }, { new Blank() });
+        }
+        
+        bool Symbol::operator==(const Rule &rule) const {
+            const Symbol *other = dynamic_cast<const Symbol *>(&rule);
+            return (other != NULL) && (other->id == id);
+        }
+        
+        Symbol * Symbol::copy() const {
+            return new Symbol(id);
+        }
+        
+        std::string Symbol::to_string() const {
+            return std::to_string(id);
+        }        
+    }
+}