v0.2 - Coverage for most of the underlying library

This adds support for much of the functionality of the underlying libiio library, including:

• Support for libiio v0.18
• Further implementation of libiio functions for contexts, devices, channels, etc.
• Functions to read and write buffers, with and without conversions, and to convert individual samples to and from hardware format.
• [Breaking] Removed previous ChannelType for Input/Output as it conflicted with the library's channel types of Voltage, Current, Power, etc, and implemented the library type.
• Contexts have a ref-counted "inner" representation using Rc<>, and can be "cloned" quickly by incrementing the count. (Thanks, @skrap!)
• Devices carry a cloned reference to the context that created them, thus keeping the context alive until the last device using it gets dropped.
• Some clippy-recommended lints.
• Example app to collect and process data a buffer at a time, with conversions.