Move the rendition vocab from default to reserved

Fixes #437

 * the 'rendition' prefix is still reserved and predefined
 * the 'rendition' prefix is allowed to be redeclared explicitly
   (with a `WARNING`)
 * the Rendition vocab URI may be associated to another prefix (with a
   `WARNING`)

Take the opportunity to better separate the vocabs computed for each
"class" of properties (`meta` & `scheme`, `item`, `itemref`, `link`).