Allow using %name as a typemap parameter.
Implement simple typemap inheritance.

After a definition of the form

  %typemap{...}{simple/reference/parsed/...}{
    %name{foo};
    ...
  };

any typemap declaration of the form

  %typemap{...}{foo}{
    ...
  };

will create a new typemap using as default the
parameters of the original typemaps
Allow overriding the default XS typemap for objects.

Defining a typemap of the form:

  %typemap{_}{simple}{
    %name{object};
    %xs_type{MY_TYPE};
    %xs_input_code{...};
    %xs_output_code{...};
  };

will override the default XS typemap for all classes;
note that the mapping will apply to all types, regardless of
the type specified in the typemap declaration.
Update documentation for x%s_type.