Add interface to move the first/last elements pointers

[reddwarf69]

I would like to use circular_buffer with Asio. And I'm not the only one -> https://stackoverflow.com/questions/19859833/read-data-into-a-circular-buffer

I could do something like

beast::buffers_cat(
asio::buffer(array_one().first, array_one().second),
asio::buffer(array_two().first, array_two().second)
)

to inform asio's async_read() of where it can write the received data to.
async_read would tell me how many bytes it has actually written into the circular_buffer.... and here I hit the wall. I need an API in circular_buffer where I can tell it "advance the pointer X elements because, even if push_back has not been called, the data is there".