Store string length

Strings are still null-terminated so that they can be treated as C strings.

src/wren_value.c

@@ -341,6 +341,7 @@ Value wrenNewUninitializedString(WrenVM* vm, size_t length)
 {
   ObjString* string = allocate(vm, sizeof(ObjString) + length + 1);
   initObj(vm, &string->obj, OBJ_STRING, vm->stringClass);
+  string->length = length;
 
   return OBJ_VAL(string);
 }

src/wren_value.h

@@ -108,6 +108,7 @@ DECLARE_BUFFER(Value, Value);
 typedef struct
 {
   Obj obj;
+  size_t length; // not including null terminator
   char value[];
 } ObjString;