[clang-tidy] replace typedef with using (#886)

Found with modernize-use-using
Signed-off-by: Rosen Penev <rosenp@gmail.com>

util/read.cpp

@@ -8,8 +8,8 @@
 
 class NullEventHandler : public YAML::EventHandler {
  public:
-  typedef YAML::Mark Mark;
-  typedef YAML::anchor_t anchor_t;
+  using Mark = YAML::Mark;
+  using anchor_t = YAML::anchor_t;
 
   NullEventHandler() = default;
 

util/sandbox.cpp

@@ -6,8 +6,8 @@
 
 class NullEventHandler : public YAML::EventHandler {
  public:
-  typedef YAML::Mark Mark;
-  typedef YAML::anchor_t anchor_t;
+  using Mark = YAML::Mark;
+  using anchor_t = YAML::anchor_t;
 
   NullEventHandler() = default;