[Code] check invalid cases when modify `fdefinition`
[Code] check invalid cases when modify `fdefinition`

These are stated in [CLHS: Accessor FDEFINITION][1].

  [1]: http://www.lispworks.com/documentation/HyperSpec/Body/f_fdefin.htm

Also, use of fdefinition of special-operators as new-definition is not
allowed, but there's no handy way to distinguish them from plain builtin-
function.

bowbow99/xyzzy.ansify