Add helper functions for making shared pointers to rules

- start work on item set class

src/grammar.cpp

@@ -0,0 +1,20 @@
+#include "grammar.h"
+
+using namespace std;
+
+namespace tree_sitter {
+    Grammar::Grammar(const rule_map &rules, const std::string &start_rule_name) :
+        rules(rules),
+        start_rule_name(start_rule_name) {};
+    
+    Grammar::Grammar(const initializer_list<string> &rule_names,
+                     const initializer_list<rules::rule_ptr> &rule_vals) {
+        rules = rule_map();
+        auto rule_name_i = rule_names.begin();
+        auto rule_i = rule_vals.begin();
+        start_rule_name = *rule_name_i;   
+        for (; rule_i != rule_vals.end(); rule_i++ && rule_name_i++) {
+            rules[*rule_name_i] = *rule_i;
+        }
+    }
+}