This PR adds a new Writer struct that can be used to write formatted strings to the LCD. The writer is constrained to a rectangle, which also makes it usable for writing scrollable or multi-column text. It supports line breaks and inserts or promotes spaces to line breaks where needed. Closes #49.

I've added an Arduino example at examples/atmega328-nostd/src/bin/formatter-4bit.rs:

// Writing to the display using ufmt
{
	let mut writer = display.writer((0, 0), (5, 1), &mut delay).unwrap();
	const HELLO_TO: &str = "world";
	ufmt::uwrite!(writer, "Hello, {}!", HELLO_TO).unwrap();

	writer = display.writer((7, 0), (15, 1), &mut delay).unwrap();
	ufmt::uwrite!(writer, "max={}", core::u32::MAX).unwrap();
}

I can add examples for more MCUs later if needed, but it's the exact same code for everything.

Is this API okay?