Crate robot_traits

This crate defined a collection of robot related traits to allow interoperability between generic inputs and outputs.

Many of the traits were originally borrowed from the python gpiozero library although I expect them to diverge over time.

Note: This is still a work in progress and the API should not be considered stable until the 1.0 release.

Traits

Led	Represents a single LED that can be turned on or off
Motor	Represents a single motor. All commands are assumed to supercede the previous one, i.e. they are not additive.
Robot	Represents a differential drive robot. All commands are assumed to supercede the previous one, i.e. they are not additive.