This makes it easier to work with device tree nodes and properties, then
writing simple state machines to parse the device tree.
This also makes the old slow traversal methods use the
DeviceTreeProperty helper class, and adds a simple test.
try_append() checks if the vector should increase capacity and if
so grows the vector. unchecked_append() verifies the vector already
has enough capacity, and will never grow the vector.
Using the walk_device_tree helper, we can walk the device tree from the
beginning looking for a specific property node. It's still missing the
ability to get property lists, string lists, and property-specific data.
