Node code: Node now has internal _set() method to set values

Summary of changes:

  * node.[pyx,pxd]: Added _set() interface

    Now __setitem__ uses _set() and the content is moved to the
    internal cdef _set() interface.

    This is a single point for setting values on dictionaries, and
    now ensures that all MappingNode keys use interned strings.

  * _yaml.pyx: Use _set() instead of accessing the MappingNode.value
               directly.