Validate fields with a trait type

[therealbstern]

Both usernames and passwords should be of specific Trait types, which incorporate validators and which are passed to the Authenticator. Examples of validation include:

• non-empty
• at least n characters long
• not in the dictionary
• contains arbitrary characters

As usual, provide default implementations of the validating types.

[therealbstern]

This is ignored by #22 but probably shouldn't be.