Drop the requirement of having only one writer and one reader, as the name of
the ring-buffer does not indicate any limitation on the thread-safety. The
two-threads-one-buffer kind of ring buffer can be reintroduced with a different
name.
Having the input type `char` makes the output of `tsrb_get_one()`
incomparable to the input of `tsrb_add_one()`. By changing it to
`uint8_t` we not only definitively fix it to an octet, but also ensure
that the input and output are the same.
