Version 0.2.0

Complete rework of the crate, most items have changed at least slightly.
Please read the updated documentation to understand how the new version
works.

Added

• A BusMutexSimple for sharing within a single task/thread with minimal overhead.
• Macros for instanciating a 'global' bus manager which lives for 'static.

Changed

• The BusMutex trait's lock() method now passes &mut to the closure, removing the RefCell from the manager.
• The generic parameter of BusMutex was moved into an associated type.
• Instead of a single proxy-type for everything, separate proxy types were introduced, to allow different constraints on their creation.

Fixed

• The SPI proxy is now !Send to make sure it can only be used from within a single thread/task.