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/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