Split Sendable into NT and non-NT portions (#3432)

The non-NT portion has been moved to wpiutil.
The NT portion has been moved to ntcore (as NTSendable).

SendableBuilder similarly split and moved.

SendableRegistry moved to wpiutil.

In C++, SendableHelper also moved to wpiutil.

This enables use of Sendable from wpimath and also enables
moving several classes from wpilib to wpimath.

wpilibc/src/main/native/include/frc/DigitalOutput.h

@@ -5,15 +5,13 @@
 #pragma once
 
 #include <hal/Types.h>
+#include <wpi/sendable/Sendable.h>
+#include <wpi/sendable/SendableHelper.h>
 
 #include "frc/DigitalSource.h"
-#include "frc/smartdashboard/Sendable.h"
-#include "frc/smartdashboard/SendableHelper.h"
 
 namespace frc {
 
-class SendableBuilder;
-
 /**
  * Class to write to digital outputs.
  *
@@ -22,8 +20,8 @@ class SendableBuilder;
  * shouldn't be done here.
  */
 class DigitalOutput : public DigitalSource,
-                      public Sendable,
-                      public SendableHelper<DigitalOutput> {
+                      public wpi::Sendable,
+                      public wpi::SendableHelper<DigitalOutput> {
  public:
   /**
    * Create an instance of a digital output.
@@ -145,7 +143,7 @@ class DigitalOutput : public DigitalSource,
    */
   void SetSimDevice(HAL_SimDeviceHandle device);
 
-  void InitSendable(SendableBuilder& builder) override;
+  void InitSendable(wpi::SendableBuilder& builder) override;
 
  private:
   int m_channel;