This can be useful in other places, e.g. it can replace objpool in
fftools.
The API is modified in the following nontrivial ways:
* opaque pointers can be passed through to all user callbacks
* read and write were previously separate callbacks in order to
accomodate the caller wishing to write a new reference to the FIFO and
keep the original one; the two callbacks are now merged into one, and
a flags argument is added that allows to request such behaviour on a
per-call basis
* new peek and drain functions
