Allow "std::byte" as "vec" element type

This is change 7 of 9 that fix problems with the specification of the
`vec` class.  An implementation that follows the existing specification
would not accept common code patterns and would not pass the CTS.  None
of the existing implementations actually follow the existing
specification.

This change clarifies that `std::byte` is a legal element type for
`vec`.  This is one possible interpretation of the previous wording
where we said that the element data type must be a "basic scalar type".
We think that phrase included `sycl::byte`.  However, `sycl::byte` was
deprecated in SYCL 2020 saying that `std::byte` is a replacement.

If we allow `std::byte`, we need to adjust the constraints for the
various `vec` operators.  We decided to allow these operators on `vec`
of `std::byte`, which mostly follows the C++ rules for operators on
plain `std::byte`:

* Comparison: ==, !=, <, <=, >, >=
* Bitwise: &, |, ^, ~, &=, |=, ^=

I decided it was clearer to rephrase the constraints to say which types
are allowed rather than listing the types that are not allowed.  For
example, this results in phrasing like:

> Available only when DataT is an integral type.

Rather than:

> Available only when: DataT != float && DataT != double &&
> DataT != half && DataT != std::byte.

These changes correspond to slide 26 of the presentation that was
discussed in the WG meetings.