Implement Send for BitValIter

[akoshelev]

Currently Iter and BitValIter have identical bodies, but only the former implements Send.

#[repr(transparent)]
#[doc = include_str!("../../doc/slice/iter/Iter.md")]
pub struct Iter<'a, T, O>
where
	T: 'a + BitStore,
	O: BitOrder,
{
	/// A dual-pointer range of the bit-slice undergoing iteration.
	///
	/// This structure stores two fully-decode pointers to the first live and
	/// first dead bits, trading increased size (three words instead of two) for
	/// faster performance when iterating.
	range: BitPtrRange<Const, T, O>,
	/// `Iter` is semantically equivalent to a `&BitSlice`.
	_ref:  PhantomData<&'a BitSlice<T, O>>,
}

...

pub struct BitValIter<'a, T, O>
where
	T: 'a + BitStore,
	O: BitOrder,
{
	/// The start and end bit-pointers in the iteration region.
	range: BitPtrRange<Const, T, O>,
	/// Hold the lifetime of the source region, so that this does not cause UAF.
	_life: PhantomData<&'a BitSlice<T, O>>,
}

It would be nice if BitValIter could also implement Send so it can be used as a direct replacement for Iter where bool values are needed