refactor: Make Span an alias of std::span

This uses a macro, which can be a bit more brittle than an alias
template. However, class template argument deduction for alias templates
is only implemented in clang-19.

doc/developer-notes.md

@@ -856,14 +856,14 @@ class A
   - *Rationale*: Easier to understand what is happening, thus easier to spot mistakes, even for those
   that are not language lawyers.
 
-- Use `Span` as function argument when it can operate on any range-like container.
+- Use `std::span` as function argument when it can operate on any range-like container.
 
   - *Rationale*: Compared to `Foo(const vector<int>&)` this avoids the need for a (potentially expensive)
     conversion to vector if the caller happens to have the input stored in another type of container.
     However, be aware of the pitfalls documented in [span.h](../src/span.h).
 
 ```cpp
-void Foo(Span<const int> data);
+void Foo(std::span<const int> data);
 
 std::vector<int> vec{1,2,3};
 Foo(vec);