Issue Description:

PropertyListEncoder should have an `encode` variant that produces the non-serialized property list, and PropertyListDecoder should have a `decode` variant that takes as input a n already-deserialized property list.

This would allow better reading/writing UserDefaults values and allow UserDefaults to do the serialization operations, and importantly, allowing the command line tool `defaults` `read` option to produce human-readable output and the `find` option to actually be able to see the property list contents.