ENH: add UList move construct and move assignment (shallow copy)

- the 'move' treatment performs a shallow copy but does not alter
  the passed parameter. Identical semantics as per std::span.

ENH: constexpr for basic HashTable constructors

STYLE: 'Foam::zero' instead of 'const Foam::zero' for containers

- this is simply a compiler dispatch flag, so the additional 'const'
  qualifier is unnecessary