v0.6.0

• DeviceInfo: (Breaking change) Return model and soc by value, rather than by reference.
• DeviceInfo: (Breaking change) Remove SoC::Bcm2837 to reduce ambiguity. The Pi 3B and Compute Module 3 now return the more accurate SoC::Bcm2837A1.
• DeviceInfo: (Breaking change) Remove SoC::Unknown. An unknown SoC is now treated as a failure.
• DeviceInfo: Return the actual SoC based on the Raspberry Pi model, rather than the inaccurate /proc/cpuinfo data.
• Gpio: (Breaking change) Remove InterruptError. Merge remaining errors with Error.
• Gpio: (Breaking change) Replace all DevGpioMem and DevMem errors with Error::PermissionDenied and Error::Io.
• Gpio: (Breaking change) Change the return value for poll_interrupt() and poll_interrupts() to Ok(Option) on success, with Some() indicating an interrupt triggered, and None indicating a timeout occurred.
• Gpio: (Breaking change) Only a single instance of Gpio can exist at any time. Creating another instance before the existing one goes out of scope will return an Error::InstanceExists.
• Spi: Add support for SPI with half-duplex reads/writes and full-duplex multi-segment transfers.