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.

wpilibc/shared/include/ErrorBase.h

@@ -113,7 +113,7 @@ class ErrorBase {
  protected:
   mutable Error m_error;
   // TODO: Replace globalError with a global list of all errors.
-  static priority_mutex _globalErrorMutex;
+  static hal::priority_mutex _globalErrorMutex;
   static Error _globalError;
 };