Remove unused boost include and split out std-hash.hh

Splitting it out immediately answers questions like [this],
without increasing the number of compilation units.

I did consider using boost::hash_combine instead, but it doesn't seem
to be quite as capable, accepting only two arguments.

[this]: https://github.com/NixOS/nix/pull/11113#discussion_r1679991573

src/libexpr/pos-idx.hh

@@ -2,7 +2,7 @@
 
 #include <cinttypes>
 
-#include "util.hh"
+#include "std-hash.hh"
 
 namespace nix {
 

src/libutil/meson.build

@@ -216,6 +216,7 @@ headers = [config_h] + files(
   'source-accessor.hh',
   'source-path.hh',
   'split.hh',
+  'std-hash.hh',
   'strings.hh',
   'strings-inline.hh',
   'suggestions.hh',

src/libutil/source-path.hh

@@ -8,8 +8,7 @@
 #include "ref.hh"
 #include "canon-path.hh"
 #include "source-accessor.hh"
-
+#include "std-hash.hh"
-#include <boost/functional/hash.hpp> // for boost::hash_combine
 
 namespace nix {
 

src/libutil/std-hash.hh

@@ -0,0 +1,24 @@
+#pragma once
+
+//!@file Hashing utilities for use with unordered_map, etc. (ie low level implementation logic, not domain logic like
+//! Nix hashing)
+
+#include <functional>
+
+namespace nix {
+
+/**
+ * hash_combine() from Boost. Hash several hashable values together
+ * into a single hash.
+ */
+inline void hash_combine(std::size_t & seed) {}
+
+template<typename T, typename... Rest>
+inline void hash_combine(std::size_t & seed, const T & v, Rest... rest)
+{
+    std::hash<T> hasher;
+    seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+    hash_combine(seed, rest...);
+}
+
+} // namespace nix

src/libutil/util.hh

@@ -375,18 +375,4 @@ inline std::string operator + (std::string_view s1, const char * s2)
     return s;
 }
 
-/**
- * hash_combine() from Boost. Hash several hashable values together
- * into a single hash.
- */
-inline void hash_combine(std::size_t & seed) { }
-
-template <typename T, typename... Rest>
-inline void hash_combine(std::size_t & seed, const T & v, Rest... rest)
-{
-    std::hash<T> hasher;
-    seed ^= hasher(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);
-    hash_combine(seed, rest...);
-}
-
 }