insert_multiple: insert multiple items into a stream

The scenario: you have an input stream, and you know you wish to insert a number of items into this stream at known offsets.

If your stream is a String or a Vec<u8> or similar, you shouldn't do this naively: performance is O(n**2), and you have to care about looping backwards through the input stream to preserve your offsets.

This crate supports this use case in O(n).