Simplify parallel GPIO interface and add new error variant

[andresovela]

I simplified the API for the parallel GPIO interface by grouping the bus into an array of pins rather than 8 individual pins. This allows to further simplify the set_value() function.

I also added a new variant to the DisplayError enum to represent issues that arise while asserting or de-asserting the reset pin. That is not used in these crates, but it is a useful error to have for consumers of this library.

[therealprof]

Thanks for the PR, it certainly looks interesting and I'll have a play with the DBUS to check how it plays with applications.

I'd like to ask you to split these two changes out into separate PRs:

• The addition of the RSError
• The change of BusWriteError -> DCError

Also please add changes to the CHANGELOG.md.

Splitting off the drivers by bumping their version seems like a good idea but I probably need to invest some time (or wait for PRs) to do it properly by having them use their own CHANGELOG.md, etc. Maybe even get rid of the workspace approach since it's still a major PITA to work with -- I totally anticipated that this would have been figured out and improved by now. 🤷🏻‍♂️

[andresovela]

I split this PR into #14, #15 and #16. I didn't bump the crate's version this time. I'll leave it to you for when you want to make another release.