Add a prelude

[jamwaffles]

There are quite a few TODOs scattered around the codebase about adding various items to a prelude to make using the crate easier. Should be easy to add a few things so that people can just add use ssd1306::prelude::*.

[jamwaffles]

So, thinking about this a bit more, I don't think a prelude is necessary. A prelude is for stuff that is indirectly required but a pain to import. Looking at all the demo code, the imports are things that are required anyway and shouldn't be hidden away behind a prelude::*.