Currently DataContext conversion is done through an as() method. The `as()` method internally delegates to an ObjectMapper.

DataContext and Castable both provide an {as()}} method. DefaultCastable provide a default implementation.

However, this makes it necessary for user classes to implement the `as()` method (usually through the DefaultCastable interface).

The as() instance method is really only for esthetic reasons – or, in other words, for lack of extension methods in Java.

A static method such as Convert.from(obj).to(target) would be just as good.

Thus:

• we remove the as() method from DataContext
• we rename DefaultCastable to DefaultConversion
• we remove the implements clause from DefaultConversion
• we delete the Castable class.

• we create the class/methods for Convert.from(obj).to(target)
• optional: DefaultConversion#as(Class<T> t) invokes Convert.from(this).to(t)

System classes will always implement DefaultConversion to provide the as() utility method. Users can opt-in whether they want to implement it or use the static version.