Rigify: added a 'copy_chain' rig type.

This is useful for bone chains that need their parent-child relationships
preserved exactly.  For example, bbone chains, and chains that users want to
use auto-ik on.
Also moved the 'copy' rig type to the basic collection.  This will break
some existing metarigs, but it is very easy to fix.  Just change the 'copy'
rig type on bones that use it to 'basic.copy'.