Add helper functions for making shared pointers to rules

- start work on item set class

src/lr/item.cpp

@@ -0,0 +1,16 @@
+#include "item.h"
+
+namespace tree_sitter {
+    namespace lr {
+        Item::Item(const std::string &rule_name, rules::rule_ptr rule, int consumed_sym_count) :
+            rule_name(rule_name),
+            rule(rule),
+            consumed_sym_count(consumed_sym_count) {};
+        
+        TransitionMap<Item> Item::transitions() const {
+            return rule->transitions().map<Item>([&](rules::rule_ptr to_rule) {
+                return item_ptr(new Item(rule_name, to_rule, consumed_sym_count + 1));
+            });
+        };
+    }
+}