Implement a delimiter for Get's bytesRead

Basically something like delimit :: Get a -> Get a that resets the bytesRead counter on the inner Get. This would primarily be useful in conjunction with the isolate combinator (which can then act undelimited by default, and we can modify it to be delimited if needed) or the alignment combinator I propose in #50.

首先定位 Get 类型及其对 bytesRead 的处理，然后阅读 isolate combinator 和 #50 中的 alignment 提案以了解上下文。将 done 定义为一个 delimiter，用于重置内部 Get 的 bytesRead 计数器，并与 isolate 以及所提议的 alignment combinator 保持一致的行为。