Fix array assert compilation failure

[chillenzer]

There's an unfortunate compiler compilation that fails to build some standard library code:

• Modern GCC >= 15 has some memory hardening that performs bounds-checking via non-constexpr asserts for spans and arrays.
• Clang device compilation relies on the standard library being constexpr for device support of the standard library.

Thus, if you try to compile standard library code for device, you might hit those asserts being non-constexpr host-only functions. LLVM >= 21.8.0 has a workaround for this (see there for details, examples, etc.) but the ROCm 6.x.x stack builds on a much older LLVM version.

This PR replaces standard library span with an own constexpr-only implementation of the necessary functionality.