Fix vstack for mixed fixed-shape inputs

[Alex-PLACET]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented.

Description

Fix xt::vstack so it works with xtensor_fixed inputs when stacking a mix of 1-D and 2-D fixed-shape expressions.

Before this change, vstack could try to build a runtime shape for fixed-shape inputs, which fails for cases like stacking xshape<1, 2> with xshape<2, 2>. The fixed-shape path now computes the stacked result shape at compile time instead.

[Alex-PLACET]

Addressing #2372

[JohanMabille]

This is not following the standard naming pattern, where usually xxx_t = typename xxx::type. Renaming vstack_fixed_shape into vstack_fixed_shape_impl, and having something like:

template <class... CT>
struct vstack_fixed_shape
{
     using type = concat_fixed_shape_t<0, typename vstack_fixed_shape_impl<typename std::decay_t<CT>::shape_type>::type...>;
};

template <class... CT>
using vstack_fixed_shape_t = typename vstack_fixed_shape_t<CT...>::type

would be more consistent with the standard