constructors: Unify C:, CONSTRUCTOR:, NAMED-CONSTRUCTOR:, and DEFAULT-CONSTRUCTOR: by requiring the class name after the constructor word name.

example: CONSTRUCTOR: <foo> foo ( stack effect -- obj ) ;

The benefit is that we can now search for <foo> by name and it's not auto-generated--less magic. Also, there are fewer kinds of constructors.

Rename CONSTRUCTOR-SYNTAX: to SLOT-CONSTRUCTOR: since it's more descriptive.
extra: Update usages of CONSTRUCTOR: for new parameter.
Link to the Python binding documentation from the Python vocabulary