Use embedded-graphics-core instead of embedded-graphics

[jamwaffles]

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

The new core crate is designed for driver use, hence this PR. Also some minor cleanup in Cargo.toml.