Intern column names so we always get the same string

When we get column names back from the database, it's very common to
always return the same strings. This patch uses Ruby's "interned string"
API so that we're always getting the same string objects back from the database.

Fixes: #155

ext/sqlite3/statement.c

@@ -382,8 +382,7 @@ column_name(VALUE self, VALUE index)
     VALUE ret = Qnil;
 
     if (name) {
-        ret = SQLITE3_UTF8_STR_NEW2(name);
-        rb_obj_freeze(ret);
+        ret = rb_enc_interned_str_cstr(name, rb_utf8_encoding());
     }
     return ret;
 }

test/test_statement.rb

@@ -48,6 +48,22 @@ def test_raises_type_error
       end
     end
 
+    def test_column_names_are_deduped
+      @db.execute 'CREATE TABLE "things" ("float" float, "int" int, "text" blob, "string" string, "nil" string)'
+      stmt = @db.prepare 'SELECT float, int, text, string, nil FROM things'
+      assert_equal ["float", "int", "text", "string", "nil"], stmt.columns
+      columns = stmt.columns
+      stmt.close
+
+      stmt = @db.prepare 'SELECT float, int, text, string, nil FROM things'
+      # Make sure this new statement returns the same interned strings
+      stmt.columns.each_with_index do |str, i|
+        assert_same columns[i], str
+      end
+    ensure
+      stmt&.close
+    end
+
     def test_insert_duplicate_records
       @db.execute 'CREATE TABLE "things" ("name" varchar(20) CONSTRAINT "index_things_on_name" UNIQUE)'
       stmt = @db.prepare("INSERT INTO things(name) VALUES(?)")