Add a thread primitive which has support for notifications.

Now you can send an `Alepha::Notification` into a thread as an
interrupt.  You have to use the Alepha threading primitives.

Thread.test/Makefile

@@ -0,0 +1,10 @@
+CXXFLAGS+= -std=c++2a -I ../
+CXXFLAGS+= -g -O0
+CXX=clang++-12
+
+CXXFLAGS+= -Wno-inline-namespace-reopened-noninline
+CXXFLAGS+= -Wno-unused-comparison
+
+LDLIBS+= -lboost_thread -lpthread
+
+all: thread

Thread.test/thread.cc

@@ -0,0 +1,70 @@
+static_assert( __cplusplus > 2020'00 );
+
+#include <Alepha/Thread.h>
+
+#include <type_traits>
+
+#include <Alepha/Testing/test.h>
+#include <Alepha/Testing/TableTest.h>
+#include <Alepha/Utility/evaluation.h>
+
+namespace
+{
+	using Alepha::exports::types::argcnt_t;
+	using Alepha::exports::types::argvec_t;
+}
+
+int
+main( const argcnt_t argcnt, const argvec_t argvec )
+{
+	return Alepha::Testing::runAllTests( argcnt, argvec );
+}
+
+namespace
+{
+	namespace util= Alepha::Utility;
+	using namespace Alepha::Testing::exports;
+
+	using MyNotification= Alepha::create_exception< struct my_notification, Alepha::Notification >;
+
+	auto tests= Alepha::Utility::enroll <=[]
+	{
+		"smoke"_test <=[] () -> bool
+		{
+			std::cerr << "Smoke started..." << std::endl;
+			Alepha::Mutex access;
+			Alepha::ConditionVariable cv;
+			auto threadMain= [&]
+			{
+				try
+				{
+					Alepha::unique_lock lock( access );
+					std::cerr << "Child thread started..." << std::endl;
+					cv.wait( lock );
+					std::cerr << "Child thread awoken illegally!" << std::endl;
+				}
+				catch( const boost::thread_interrupted & )
+				{
+					std::cerr << "SHIT!  We didn't get intercepted!" << std::endl;
+					throw;
+				}
+				catch( const MyNotification &n )
+				{
+					std::cerr << "I caught it: " << n.message() << "!" << std::endl;
+					throw;
+				}
+			};
+
+			access.lock();
+			std::cerr << "Launching child thread..." << std::endl;
+			Alepha::Thread thr( threadMain );
+			std::cerr << "Child thread now launched..." << std::endl;
+			::sleep( 1 );
+			access.unlock();
+			thr.interrupt( Alepha::build_exception< MyNotification >( "My message" ) );
+			thr.join();
+
+			return true;
+		};
+	};
+}