Minor improvements to 5.4.0-beta1

[ColinFinck]

Removes no longer required dependencies and fix some comments.

Apart from that, 5.4.0-beta1 looks fine to me. Did you really want to release that as 5.4.0 though and not 4.4.0 or 5.0?
I guess, you could still yank the 5.4.0-beta1 release from crates.io and change the version number if required.

[berkowski]

Breaking API changes should require a major version bump if playing by semver rules. Ideally I'd use v4.4.0 for this, but v4.3 was already released with the old API.

[ColinFinck]

Fair enough. 5.4.0 then or 5.0.0?

[berkowski]

The old version scheme was $MIO_SERIAL_MAJOR.$SERIALPORT-RS_MAJOR.$TOKIO_SERIAL_PATCH as the tokio stuff is really just a wrapper around mio, and the mio API is heavily influenced by serialport-rs.

I guess if I want to stick to this I could skip a v4 release of mio-serial and go direct to v5 which would keep the versions more in sync

[ColinFinck]

Now I understand 5.4.0, thanks. Is this PR alright?

[berkowski]

PR looks good pending the small change named in my comment. I've been holding of merging it until I finish bashing my head against CI test infrastructure on mio-serial