{libutil,libexpr}: Move pos-idx,pos-table code to libutil

All of this code doesn't actually depend on anything from
libexpr. Because Pos is so tigtly coupled with Error, it
makes sense to have in the same library.

(cherry picked from commit a53b184e63)

src/libutil/pos-idx.hh

@@ -0,0 +1,65 @@
+#pragma once
+///@file
+
+#include <cinttypes>
+#include <functional>
+
+namespace nix {
+
+class PosIdx
+{
+    friend struct LazyPosAcessors;
+    friend class PosTable;
+    friend class std::hash<PosIdx>;
+
+private:
+    uint32_t id;
+
+    explicit PosIdx(uint32_t id)
+        : id(id)
+    {
+    }
+
+public:
+    PosIdx()
+        : id(0)
+    {
+    }
+
+    explicit operator bool() const
+    {
+        return id > 0;
+    }
+
+    auto operator<=>(const PosIdx other) const
+    {
+        return id <=> other.id;
+    }
+
+    bool operator==(const PosIdx other) const
+    {
+        return id == other.id;
+    }
+
+    size_t hash() const noexcept
+    {
+        return std::hash<uint32_t>{}(id);
+    }
+};
+
+inline PosIdx noPos = {};
+
+}
+
+namespace std {
+
+template<>
+struct hash<nix::PosIdx>
+{
+    std::size_t operator()(nix::PosIdx pos) const noexcept
+    {
+        return pos.hash();
+    }
+};
+
+} // namespace std