Add `isstr` to type-checking collection

src/env.c

@@ -229,7 +229,7 @@ struct Environment defaultEnv()
         {"cat", &catObjects}, {"fil", &filter}, {"len", &len},
         {"ap", &append}, {"pre", &prepend},
         {"at", &at}, {"rest", &rest}, {"rev", &reverse},
-        {"isnum", &isNum},
+        {"isnum", &isNum}, {"isstr", &isString},
         {"prn", &print}, {"penv", &printEnvO},
     };
 

src/pebblisp.c

@@ -392,6 +392,12 @@ Object isNum(Object test, Object ignore, struct Environment *ignore2)
 		boolObject(1) : boolObject(0);
 }
 
+Object isString(Object test, Object ignore, struct Environment *ignore2)
+{
+	return test.type == TYPE_STRING ?
+		boolObject(1) : boolObject(0);
+}
+
 Object print(Object p, Object ignore, struct Environment *env)
 {
     p = cloneObject(p);

src/pebblisp.h

@@ -56,6 +56,7 @@ Object rest(Object list, Object ignore, struct Environment *env);
 Object reverse(Object _list, Object ignore, struct Environment *ignore2);
 
 Object isNum(Object test, Object ignore, struct Environment *ignore2);
+Object isString(Object test, Object ignore, struct Environment *ignore2);
 
 Object print(Object p, Object ignore, struct Environment *ignore2);
 Object printEnvO(Object i1, Object i2, struct Environment *env);