Renaming to Rest, Resource is now a record

The type classes dealing with stored are now unified into a record of
Maybe functions, and can be constructed using the lens setter interface.
In the future the builder interface should be extended to ensure that
either exists or fetch are present when update or delete are
implemented.
Removing OPTIONS body, sending Allow header

This is a partial implementation of OPTIONS, pending a comprehensive
HATEOAS solution.
Using fetch instead of exists for HEAD