add get_pixel & toggle_pixel & extract shared code

Hi! Thank you for helping out with SSD1306 development! Please:

[ ] Check that you've added documentation to any new methods
[ ] Rebase from master if you're not already up to date
[ ] Add or modify an example if there are changes to the public API
[ ] Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
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PR description

alternative to https://github.com/jamwaffles/ssd1306/pull/193 that also contains a toggle_pixel function.