ranges: add a simplistic implementation of enumerate

This is a temporary workaround because clang lacks it.

cruft/util/CMakeLists.txt

@@ -545,6 +545,7 @@ list (
     range.cpp
     range.hpp
     ranges/chunk.hpp
+    ranges/enumerate.hpp
     rational.cpp
     rational.hpp
     region.cpp

cruft/util/ranges/enumerate.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <ranges>
+
+namespace cruft::util::ranges {
+    /// A very simple implementation of std::views::enumerate provided because clang-19 lacks it
+    ///
+    /// clang#19: check me again in clang-20
+    template <typename Rng>
+    requires (std::ranges::input_range<Rng>)
+    auto
+    enumerate (Rng &&r)
+    {
+        return std::views::zip (
+            std::views::iota (0),
+            std::forward<Rng> (r)
+        );
+    }
+}