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.

Start by locating the Get type and its bytesRead handling, then read the isolate combinator and the alignment proposal in #50 for context. Define done as a delimiter that resets the inner Get's bytesRead counter and behaves consistently with isolate and the proposed alignment combinator.

Written by the indexing model from the issue text.