Suggestion: add 2 differents lifetime for buffer_in and buffer_out in `process_into_buffer`

[wiiznokes]

It was causing some borrowing issue in my code

[HEnquist]

Good idea, there is no reason to tie the lifetimes of input and output together.
It's a breaking change so will have to wait for the next major version. I don't have any other changes planned at the moment though. Let's see if anything else shows up soon, otherwise I'll make a new major version with only this change.