Convert operator+(Population, Population) into a hidden friend

tools/yulPhaser/Population.cpp

@@ -118,15 +118,21 @@ Population Population::crossover(PairSelection const& _selection, function<Cross
 	return Population(m_fitnessMetric, crossedIndividuals);
 }
 
+namespace solidity::phaser
+{
+
 Population operator+(Population _a, Population _b)
 {
 	// This operator is meant to be used only with populations sharing the same metric (and, to make
 	// things simple, "the same" here means the same exact object in memory).
 	assert(_a.m_fitnessMetric == _b.m_fitnessMetric);
 
+	using ::operator+; // Import the std::vector concat operator from CommonData.h
 	return Population(_a.m_fitnessMetric, move(_a.m_individuals) + move(_b.m_individuals));
 }
 
+}
+
 bool Population::operator==(Population const& _other) const
 {
 	// We consider populations identical only if they share the same exact instance of the metric.