BufferedIterator wraps an existing iterable and allows the consumer to

peek contents without consuming the iterable. It's particularly useful

for parsing output from commands or diff generation, and to allow more

efficient chained processing.
This is implemented as an iterator wrapping another iterator, with a

deque used to hold the buffer. When peeking, we ensure the buffer has

been populated with the number of items to peek and we return from

there. When consuming or iterating, we start by retrieving items from

the buffer before retrieving from the iterator.

Unit tests pass.
Used this with some in-progress work for chained diff processing.