Merge bitcoin/bitcoin#35895: refactor: Enable clang-tidy rule to reject anon namespace in header

### src/.clang-tidy
@@ -8,8 +8,10 @@ bugprone-use-after-move,
 bugprone-lambda-function-name,
 bugprone-unhandled-self-assignment,
 bugprone-unused-return-value,
-misc-unused-using-decls,
+fuchsia-header-anon-namespaces,
+misc-definitions-in-headers,
 misc-no-recursion,
+misc-unused-using-decls,
 modernize-avoid-bind,
 modernize-deprecated-headers,
 modernize-use-default-member-init,

### src/bench/nanobench.h
@@ -1369,6 +1369,7 @@ void doNotOptimizeAway(T const& val) {
 } // namespace ankerl
 
 #if defined(ANKERL_NANOBENCH_IMPLEMENT)
+// NOLINTBEGIN(misc-definitions-in-headers)
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 // implementation part - only visible in .cpp
@@ -3563,5 +3564,6 @@ std::ostream& operator<<(std::ostream& os, std::vector<ankerl::nanobench::BigO>
 } // namespace nanobench
 } // namespace ankerl
 
+// NOLINTEND(misc-definitions-in-headers)
 #endif // ANKERL_NANOBENCH_IMPLEMENT
 #endif // ANKERL_NANOBENCH_H_INCLUDED

### src/util/strencodings.h
@@ -17,6 +17,7 @@
 #include <charconv>
 #include <cstddef>
 #include <cstdint>
+#include <functional>
 #include <limits>
 #include <optional>
 #include <span>
@@ -214,18 +215,10 @@ bool TimingResistantEqual(const T& a, const T& b)
  */
 [[nodiscard]] bool ParseFixedPoint(std::string_view, int decimals, int64_t *amount_out);
 
-namespace {
-/** Helper class for the default infn argument to ConvertBits (just returns the input). */
-struct IntIdentity
-{
-    [[maybe_unused]] int operator()(int x) const { return x; }
-};
-
-} // namespace
-
 /** Convert from one power-of-2 number base to another. */
-template<int frombits, int tobits, bool pad, typename O, typename It, typename I = IntIdentity>
-bool ConvertBits(O outfn, It it, It end, I infn = {}) {
+template <int frombits, int tobits, bool pad, typename O, typename It, typename I = std::identity>
+bool ConvertBits(O outfn, It it, It end, I infn = {})
+{
     size_t acc = 0;
     size_t bits = 0;
     constexpr size_t maxv = (1 << tobits) - 1;