Add some abstractions to simplify the `rbuf_but_last_n_lines()`
function, which implements the "take all but the last `n` lines"
functionality of the `head` program. This commit adds
- `RingBuffer`, a fixed-size ring buffer,
- `ZLines`, an iterator over zero-terminated "lines",
- `TakeAllBut`, an iterator over all but the last `n` elements of an
iterator.
These three together make the implementation of
`rbuf_but_last_n_lines()` concise.
Refactor code from the `backwards_thru_file()` function into a new
`ReverseChunks` iterator, and use that iterator to simplify the
implementation of the `backwards_thru_file()` function. The
`ReverseChunks` iterator yields `Vec<u8>` objects, each of which
references bytes of a given file.
Refactor common code out of two branches of the `unbounded_tail()`
function into a new `unbounded_tail_collect()` helper function, that
collects from an iterator into a `VecDeque` and keeps either the last
`n` elements or all but the first `n` elements.
This commit also adds a new struct, `RingBuffer`, in a new module,
`ringbuffer.rs`, to be responsible for keeping the last `n` elements
of an iterator.
- refactor internal version specifications to be ">=M.m.p" (where M.m.p is *already published*)
## [why]
Loosening internal version dependencies decreases the coupling between packages such
that packages can be published in a looser order. It allows the packages to be version
updated and published in tandem (ie, by using `cargo workspace ...`). Once published,
the internal versions can then be updated (again, to an *already published* package
version), as needed.
