Officially document sugar-free form for type and subtype where it

Officially document sugar-free form for type and subtype where it
accepts a hashref of named parameters, rather than positional params.

This preserves the safety of non-positional params, but lets you
integrate easily with other modules, for doing

  subtype( 'Foo', { where => sub_returner() } );

Added tests for this new style and made sure the docs show it too.