Initial WB55 support.

[rubdos]

• Where does the SVD come from?
• feature flags on all relevant peripherals (is there a way to automate this?)

Accompanies drone-os/drone#1

[valff]

In my experience there are always many small differences in peripherals across STM32 families. I doubt it will even compile if we enable them all. I propose to start without any peripheral (in Drone terms) mappings. So this crate will initially provide just register and interrupt mappings.

[valff]

If you wonder, we use so many crates here because they dramatically reduce compile times. Register mappings generation parallelizes nicely.

[valff]

To clarify, periph-* crates just group registers by peripherals. For example this allows you to write code generic over UART1, UART2, ..., LPUART1. The problem is that often these peripherals have small differences, so they should be addressed by peripheral crates.