ClangTidy cleanups

1. NULL -> nullptr
2. foo.reset(new Bar) -> = foo = std::make_unique<Bar>()
3. Missing std library includes & prefixes

Change-Id: I260b261b484554be681ee5a7398126fdb3b3a789
diff --git a/internal/ceres/covariance.cc b/internal/ceres/covariance.cc
index ead3287..d63dd37 100644
--- a/internal/ceres/covariance.cc
+++ b/internal/ceres/covariance.cc
@@ -39,12 +39,11 @@
 
 namespace ceres {
 
-using std::make_pair;
 using std::pair;
 using std::vector;
 
 Covariance::Covariance(const Covariance::Options& options) {
-  impl_.reset(new internal::CovarianceImpl(options));
+  impl_ = std::make_unique<internal::CovarianceImpl>(options);
 }
 
 Covariance::~Covariance() = default;