Let's write a UART driver

The QEMU 'virt' machine has a PL011 UART, so let's write a driver for that.

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
{{#include examples/src/pl011_minimal.rs:Example}}

• Note that Uart::new is unsafe while the other methods are safe. This is because as long as the caller of Uart::new guarantees that its safety requirements are met (i.e. that there is only ever one instance of the driver for a given UART, and nothing else aliasing its address space), then it is always safe to call write_byte later because we can assume the necessary preconditions.
• We could have done it the other way around (making new safe but write_byte unsafe), but that would be much less convenient to use as every place that calls write_byte would need to reason about the safety
• This is a common pattern for writing safe wrappers of unsafe code: moving the burden of proof for soundness from a large number of places to a smaller number of places.