Add traits for nodes and properties

[qwandor]

Node is implemented by both FdtNode and &DeviceTreeNode, while Property is implemented by FdtProperty and &DeviceTreeProperty. These traits have required methods to look up properties, children, and so on, and then provide methods to get standard nodes and properties.

TODO:

• Clean up documentation and examples
• See if implementing Deref or something for DeviceTreeNode can make calling methods from the Node trait on it nicer.

[m4tx]

The old discussion for context: #16 (comment)

I thought of yet another way to solve the code duplication problem. Perhaps we could change the intermediate representation API a little bit to match Fdt more:

• We could change DeviceTreeNode and DeviceTreeProperty to become internal structures
• Similarly to FdtNode and FdtProperty, we could create similar structures for the DeviceTree API that would borrow the data from DeviceTree
• Adding (and perhaps modifying) data could be done using builder structures

This would solve the problems with the trait trickery because the lifetimes would be the same for both APIs, and would probably simplify things with the Reg property getter, for instance. Having a very similar API between in-memory FDT, and our internal representation sounds like an added bonus that simplifies things for the users.

[qwandor]

Inspired by your comment, it turns out there's a simpler way: I can implement the trait for &DeviceTreeNode rather than for DeviceTreeNode. That way the lifetimes all seem to work out so far.