dtlib: code order and whitespace refactoring

Move the DTError, Node, Type, and Property definitions to the top.

This way, class definitions occur before methods which use those
classes. This will be useful to avoid string literals in type
annotations that will be added later. Some can't be avoided due to
circular dependencies, but this will help.

Adjust whitespace.

No functional changes expected.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>