renamed rbuf to rqueue since it's more a ring queue than a ringbuffer

it allows to store/read pointers using a ringbuffer but it doesn't allow storing
actual bytes and it's just an alternative strategy to handle queues.
Hence tha new name : rqueue (as for ring-queue)
renamed 'rbb' to 'rbuf' since this is an actual ringbuffer, as opposed to the flat buffer implementation 'fbuf'

added tests as well
privatized the fbuf_t structure