[Stream.h] make MultiTarget and findMulti public

Why MultiTarget and findMulti in Stream.h are not public members of Stream? Their implementation is by no means trivial and they seem relatively safe, so why hide this functionality from users?
I found it very useful e.g. for parsing text commands coming from serial port. I'm sure there's a lot of other cases where it may help.

从 Stream.h 开始，定位 MultiTarget 和 findMulti 的声明与实现，然后检查当前如何定义它们的可见性。完成的标准是两个成员都可以公开访问，同时不改变其现有行为；检查周围的 Stream API 和可用测试以确保兼容性。

由索引模型根据 Issue 内容生成。