Reading from the LCD

[ColinTimBarndt]

I'm working on the implementations for reading data from the display. For now, I've only implemented it for blocking I2C. To do this, I split DataBus into two traits for reading and writing because depending on the pin configuration, reading may not be possible (and is not required for basic operation).

Notable Changes

• rw pin was added to 4/8-bit bus pin structs. Instead of a pin, WriteOnlyMode can be specified to opt-out of read features. This requires pulling the RW pin low on the display. This is a breaking change, but easy to adapt.
• DataBus trait was split into Read and Write traits.
• Added read_status

[ColinTimBarndt]

This PR is ready for review. Should we support reading back character data from the display? I don't really see a reason for doing so, but it would be trivial to implement