Currently, when declaring a field on a Mongoid::Document, you must specify the type as either a symbol (for a limited set of built-in types) or by using the class directly.

Referencing the class name directly leads to chicken-vs-egg issues (if there are circular dependencies, for example).

A clean way of dealing with this is to allow types to also be strings, which name the class. We already support this with "Boolean" as a special case (a shorthand for Mongoid::Boolean). I propose we make this a more general solution as well. It is also a much simpler solution than (e.g.) MONGOID-5210.