More convenient shorthand for derived attributes

Resolving #165 allowed setting the default for an attribute based on other attributes using a decorator like this:

@attrs(frozen=True, slots=True)
class Foo:
   bar = attr.ib()
   moo = attr.ib()

   @moo.default
   def _init_moo(self):
      return self.bar + 2

Many times in frozen classes we want to derive one attribute from another without giving the user the opportunity to override it. The Immutables library for Java provides for this via Value.Derived: http://immutables.github.io/immutable.html#derived-attributes . Perhaps attrs could have a @field.derived decorator which does the same thing?

Start by reading the existing @moo.default decorator behavior and how attrs handles frozen classes. Compare that with the proposed @field.derived entry point and the linked Immutables example. Done means a derived attribute can be calculated from other attributes without allowing the user to override it, with behavior and scope agreed for attrs.