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.

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