caos: Turn builtins into a proper module
caos: Turn builtins into a proper module

Instead of adding builtins by hand, create a proper module
'semantix.caos.builtins' that hosts all fundamental objects: atoms, and
base concept and link.

This effectively removes almost all special-handling of builtin objects,
e.g. the base tables are now created for them just as for regular
objects.

This commit also brings previously missing base link object:
semantix.caos.builtins.link from which all links inherit.

Another feature of the rework is the ability to specify an arbitrary
Python class as a base for an atom, as all builtin atoms use that
feature.

sprymix/importkit