Add support for defer_lock in nano::unique_lock (#2909)

Needs NANO_TIMED_LOCKS CMake variable > 0 to test, otherwise it defaults
to using std::unique_lock

This is necessary to build since #2901 but not caught by CI due to not
using this developer-oriented feature

nano/core_test/locks.cpp

@@ -130,4 +130,15 @@ TEST (locks, condition_variable)
 	finished = true;
 	t.join ();
 }
+
+TEST (locks, defer_lock)
+{
+	std::mutex mutex;
+	nano::unique_lock<std::mutex> lock (mutex, std::defer_lock);
+	ASSERT_FALSE (lock.owns_lock ());
+	ASSERT_TRUE (lock.try_lock ());
+	ASSERT_TRUE (lock.owns_lock ());
+	lock.unlock ();
+	ASSERT_FALSE (lock.owns_lock ());
+}
 #endif

nano/lib/locks.cpp

@@ -66,6 +66,12 @@ mut (std::addressof (mutex))
 	lock_impl ();
 }
 
+template <typename Mutex, typename U>
+unique_lock<Mutex, U>::unique_lock (Mutex & mutex, std::defer_lock_t) noexcept :
+mut (std::addressof (mutex))
+{
+}
+
 template <typename Mutex, typename U>
 void unique_lock<Mutex, U>::lock_impl ()
 {

nano/lib/locks.hpp

@@ -46,6 +46,7 @@ class unique_lock final
 public:
 	unique_lock () = default;
 	explicit unique_lock (Mutex & mutex_a);
+	unique_lock (Mutex & mutex_a, std::defer_lock_t) noexcept;
 	unique_lock (unique_lock && other) = delete;
 	unique_lock & operator= (unique_lock && other) noexcept;
 	~unique_lock () noexcept;