feat(attributes): Support nested attributes

[nicholasamorim]

Implement work from pyradius/pyrad#206

• Adds nesting to dictionary attributes and packet values
• sub-attributes are stored as children of their parent attributes
• sub-attributes are not visible in the global namespace, only their parents
• vendor attributes are children of vendors, which are children of the Vendor-Specific attribute
• Drop many top-levels functions in tools.py and replace them with datatypes classes in datatypes.types