Moves the HAL priority_ custom types to the hal namespace (#532)

There is a shim for backwards compatibility, just like the frc namespace.
As with the frc namespace, the library compiles without the shim.

hal/include/HAL/cpp/SerialHelper.h

@@ -45,7 +45,7 @@ class SerialHelper {
 
   int32_t m_resourceHandle;
 
-  static priority_mutex m_nameMutex;
+  static hal::priority_mutex m_nameMutex;
   static std::string m_usbNames[2];
 };
 }  // namespace hal

hal/include/HAL/cpp/priority_condition_variable.h

@@ -22,6 +22,8 @@
 
 #include "priority_mutex.h"
 
+namespace hal {
+
 class priority_condition_variable {
   typedef std::chrono::system_clock clock;
 
@@ -130,3 +132,10 @@ class priority_condition_variable {
     Lock& m_lock;
   };
 };
+
+}  // namespace hal
+
+// For backwards compatibility
+#ifndef NAMESPACED_PRIORITY
+using priority_condition_variable = hal::priority_condition_variable;  // NOLINT
+#endif

hal/include/HAL/cpp/priority_mutex.h

@@ -11,14 +11,18 @@
 #include <mutex>
 
 #if defined(FRC_SIMULATOR) || defined(_WIN32)
+namespace hal {
 // We do not want to use pthreads if in the simulator; however, in the
 // simulator, we do not care about priority inversion.
 typedef std::mutex priority_mutex;
 typedef std::recursive_mutex priority_recursive_mutex;
+}  // namespace hal
 #else  // Covers rest of file.
 
 #include <pthread.h>
 
+namespace hal {
+
 class priority_recursive_mutex {
  public:
   typedef pthread_mutex_t* native_handle_type;
@@ -77,5 +81,12 @@ class priority_mutex {
   pthread_mutex_t m_mutex = {{0, 0, 0, 0x20, 0, {0}}};
 #endif
 };
+}  // namespace hal
 
 #endif  // FRC_SIMULATOR
+
+// For backwards compatibility
+#ifndef NAMESPACED_PRIORITY
+using priority_mutex = hal::priority_mutex;                      // NOLINT
+using priority_recursive_mutex = hal::priority_recursive_mutex;  // NOLINT
+#endif