Avoid GCC -Warray-bounds false positive on fixed-size shape copies (#2946)

# Description

GCC 13 and 15 emit `-Warray-bounds` at `-O2`/`-O3` on ordinary code
using fixed-rank containers, for example assigning `0.5 * (even + odd)`
to a 1-D `xtensor` view. The warning comes from a `std::copy` between
two `std::array<std::size_t, 1>` when the source is a member at a
non-zero offset. GCC does not fold the length of the range and warns on
the `memmove` branch that libstdc++ keeps for ranges of more than one
element. That branch never runs. Reproducer without xtensor:

```cpp
struct holder { std::size_t m_size; std::array<std::size_t, 1> m_shape; };
std::array<std::size_t, 1> f(const holder& h)
{
    std::array<std::size_t, 1> s;
    std::copy(h.m_shape.cbegin(), h.m_shape.cend(), s.begin());
    return s;
}
```

These copies now use `std::copy_n(src.begin(), src.size(),
dst.begin())`. `size()` is a constant for `std::array`, so GCC drops the
branch. At each site the destination is sized from the same source, so
the copied elements do not change. Changed sites:
`xfunction::broadcast_shape`, the `xbroadcast` constructor, `flip`
(shape and strides), and `average` with multi-dimensional weights.
`std::copy(data(), data() + size())` and `std::ranges::copy` still warn.

No unit test, since this is a compile-time warning and CI does not build
with `-Wall` on GCC. A file using 1-D, 2-D and 3-D views, reductions,
broadcasting, `flip` and `xtensor_fixed` now compiles without warnings
with GCC 13.4 and 15.3 at `-O2` and `-O3` with `-Wall -Wextra`, with and
without xsimd.

Author: gouarin

include/xtensor/core/xfunction.hpp

@@ -712,7 +712,7 @@ namespace xt
     {
         if (m_cache.is_initialized && reuse_cache)
         {
-            std::copy(m_cache.shape.cbegin(), m_cache.shape.cend(), shape.begin());
+            std::copy_n(m_cache.shape.cbegin(), m_cache.shape.size(), shape.begin());
             return m_cache.is_trivial;
         }
         else

include/xtensor/core/xmath.hpp

@@ -1960,7 +1960,7 @@ namespace xt
                 );
             }
 
-            std::copy(e.shape().begin(), e.shape().end(), broadcast_shape.begin());
+            std::copy_n(e.shape().begin(), e.shape().size(), broadcast_shape.begin());
         }
 
         constexpr layout_type L = default_assignable_layout(std::decay_t<W>::static_layout);

include/xtensor/misc/xmanipulation.hpp

@@ -840,12 +840,12 @@ namespace xt
 
         shape_type shape;
         resize_container(shape, e.shape().size());
-        std::copy(e.shape().cbegin(), e.shape().cend(), shape.begin());
+        std::copy_n(e.shape().cbegin(), e.shape().size(), shape.begin());
 
         get_strides_t<shape_type> strides;
         decltype(auto) old_strides = detail::get_strides<XTENSOR_DEFAULT_LAYOUT>(e);
         resize_container(strides, old_strides.size());
-        std::copy(old_strides.cbegin(), old_strides.cend(), strides.begin());
+        std::copy_n(old_strides.cbegin(), old_strides.size(), strides.begin());
 
         strides[axis] *= -1;
         std::size_t offset = static_cast<std::size_t>(

include/xtensor/views/xbroadcast.hpp

@@ -293,7 +293,7 @@ namespace xt
             XTENSOR_THROW(xt::broadcast_error, "Broadcast shape has fewer elements than original expression.");
         }
         xt::resize_container(m_shape, s.size());
-        std::copy(s.begin(), s.end(), m_shape.begin());
+        std::copy_n(s.begin(), s.size(), m_shape.begin());
         xt::broadcast_shape(m_e.shape(), m_shape);
     }