Rename next_{terminals,non_terminals} to first_set and follow_sets

This is to prepare for keeping track of lookahead symbols as part
of computing follow sets

src/compiler/build_tables/first_set.cpp

@@ -0,0 +1,65 @@
+#include "first_set.h"
+#include "rule_transitions.h"
+#include "grammar.h"
+#include <vector>
+
+using std::set;
+using std::vector;
+using std::dynamic_pointer_cast;
+using namespace tree_sitter::rules;
+
+namespace tree_sitter {
+    namespace build_tables {
+        class FirstSetVisitor : Visitor {
+            set<Symbol> value;
+            const Grammar grammar;
+            
+            FirstSetVisitor(const Grammar &grammar) : grammar(grammar) {}
+            
+            set<Symbol> set_union(const set<Symbol> &left, const set<Symbol> &right) {
+                set<Symbol> result = left;
+                result.insert(right.begin(), right.end());
+                return result;
+            }
+            
+            void visit(const Symbol *rule) {
+                if (grammar.has_definition(*rule)) {
+                    value = apply(grammar.rule(rule->name), grammar);
+                } else {
+                    value = set<Symbol>({ *rule });
+                }
+            }
+            
+            void visit(const Choice *rule) {
+                value = set_union(apply(rule->left, grammar), apply(rule->right, grammar));
+            }
+            
+            void visit(const Seq *rule) {
+                if (rule_can_be_blank(rule->left)) {
+                    value = set_union(apply(rule->left, grammar), apply(rule->right, grammar));
+                } else {
+                    value = apply(rule->left, grammar);
+                }
+            }
+
+        public:
+            static set<Symbol> apply(const rule_ptr rule, const Grammar &grammar) {
+                FirstSetVisitor visitor(grammar);
+                rule->accept(visitor);
+                return visitor.value;
+            }
+        };
+        
+        set<rules::Symbol> first_set(const rules::rule_ptr &rule, const Grammar &grammar) {
+            return FirstSetVisitor::apply(rule, grammar);
+        }
+
+        set<rules::Symbol> first_set(const ParseItemSet &item_set, const Grammar &grammar) {
+            set<rules::Symbol> result;
+            for (auto item : item_set)
+                for (rules::Symbol symbol : first_set(item.rule, grammar))
+                    result.insert(symbol);
+            return result;
+        }
+    }
+}