This introduces a couple of specialized properties that help with

deprecating older APIs and enforcing type safety with minimal effort.
AliasProperty forwards on any property access (sets and gets) to

another attribute, optionally converting the new or existing value, and

optionally emitting a deprecation warning.
TypedProperty takes one or more value types that are allowed to be

set, and checks any new values against those types, optionally allowing

or disallowing None values.
These make use of a new function, get_descriptor_attr_name(), which

can locate the attribute of a descriptor/property in a class and return

the attribute name. It's used by these properties for error reporting

and for setting an attribute on a class instance based on the property's

name.

Unit tests passed.
Made use of these in some upcoming changes.