Document and verify that emacs_limb_t doesn’t have padding bits.

This is a useful property when doing further bit-twiddling with the
magnitude array before/after calling extract_big_integer or
make_big_integer.  For example, constructing an emacs_limb_t object
using repeated shift-and-add should work as expected, but relies on
the type not having padding bits.  Since the C standard already
guarantees that unsigned integers use a pure binary representation,
not having padding bits is enough to guarantee that the type has
unique object representations in the sense of C++’s
std::has_unique_object_representations.

* doc/lispref/internals.texi (Module Values): Document that
emacs_limb_t doesn’t have padding bits.

* src/emacs-module.c: Verify that emacs_limb_t doesn’t have padding
bits.

doc/lispref/internals.texi

@@ -1508,9 +1508,10 @@ overflow in the size calculation.
 @end deftypefn
 
 @deftp {Type alias} emacs_limb_t
-This is an unsigned integer type,
-used as the element type for the magnitude arrays for the big
-integer conversion functions.
+This is an unsigned integer type, used as the element type for the
+magnitude arrays for the big integer conversion functions.  The type
+is guaranteed to have unique object representations, i.e., no padding
+bits.
 @end deftp
 
 @defvr Macro EMACS_LIMB_MAX

src/emacs-module.c

@@ -827,6 +827,13 @@ enum
   module_bignum_count_max = min (SIZE_MAX, PTRDIFF_MAX) / sizeof (emacs_limb_t)
 };
 
+/* Verify that emacs_limb_t indeed has unique object
+   representations.  */
+verify (CHAR_BIT == 8);
+verify ((sizeof (emacs_limb_t) == 4 && EMACS_LIMB_MAX == 0xFFFFFFFF)
+        || (sizeof (emacs_limb_t) == 8
+            && EMACS_LIMB_MAX == 0xFFFFFFFFFFFFFFFF));
+
 static bool
 module_extract_big_integer (emacs_env *env, emacs_value arg, int *sign,
                             ptrdiff_t *count, emacs_limb_t *magnitude)