String move() and String(String &&rval) breaks operation of reserve()

When #if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
operator = uses move() to just update the buffer pointer of the destination
This ignores any reserve() the user has made to ensure the memory is not unnecessarily fragmented.
String(String &&rval) has a similar problem

move() should first check the capacity of the destination and if there is sufficient space copy the source to the destination
String(String &&rval) should use move()

A suggested move() is

```
void String::move(String &rhs) {
if (this != &rhs) {
if (capacity > rhs.size) {
copy(rhs.buffer,rhs.size);
} else {
free(buffer);
buffer = rhs.buffer;
len = rhs.len;
capacity = rhs.capacity;
}
rhs.buffer = NULL;
rhs.len = 0;
rhs.capacity = 0;
}
}
```

找到 C++11 String 实现，并检查 move()、operator=、String(String &&rval) 构造函数以及 reserve() 的行为。验证 reserved destination 在 move 期间的行为，然后确认构造函数遵循预期的 move path，并且 moved-from strings 仍保持有效。

由索引模型根据 Issue 内容生成。