node.python: make link's ID a 'default' key for misc operations, update and fix bindings
node.python: make link's ID a 'default' key for misc operations, update and fix bindings

In eralier implentation a key passed to Python's operators was
interpreted as link's name or `objbase` instance. It turns out that
more convinient is to have link's ID as such 'default' index.
Also export `index()` functions and correct bindings of `find()`, etc.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re