Trac #33193: redeclare val_unit() in padic subclasses that override it.

When overriding a C method to add optional parameters in Cython, you
have to re-declare the method in the corresponding subclass pxd file;
otherwise, you'll get warnings like those that Cython is throwing:

  warning: sage/rings/padics/CR_template.pxi:1478:10: Compatible but
  non-identical C method 'val_unit' not redeclared in definition part
  of extension type 'CRElement'.  This may cause incorrect vtables to
  be generated.

In this case, things are a bit convoluted due to templates being used,
but the solution still boils down to re-declaring the subclass methods
with the additional optional argument.

src/sage/rings/padics/CR_template_header.pxi

@@ -34,6 +34,7 @@ cdef class CRElement(pAdicTemplateElement):
 
     cdef CRElement _new_c(self)
     cdef int _normalize(self) except -1
+    cpdef val_unit(self, p=*)
 
 cdef class pAdicCoercion_ZZ_CR(RingHomomorphism):
     cdef CRElement _zero

src/sage/rings/padics/FP_template_header.pxi

@@ -34,6 +34,7 @@ cdef class FPElement(pAdicTemplateElement):
     cdef FPElement _new_c(self)
     cdef int _normalize(self) except -1
     cdef int _set_infinity(self) except -1
+    cpdef val_unit(self, p=*)
 
 cdef class pAdicCoercion_ZZ_FP(RingHomomorphism):
     cdef FPElement _zero