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/AnalogAccelerometer.h

@@ -6,14 +6,13 @@
 
 #include <memory>
 
+#include <wpi/sendable/Sendable.h>
+#include <wpi/sendable/SendableHelper.h>
+
 #include "frc/AnalogInput.h"
-#include "frc/smartdashboard/Sendable.h"
-#include "frc/smartdashboard/SendableHelper.h"
 
 namespace frc {
 
-class SendableBuilder;
-
 /**
  * Handle operation of an analog accelerometer.
  *
@@ -21,8 +20,8 @@ class SendableBuilder;
  * sensors have multiple axis and can be treated as multiple devices. Each is
  * calibrated by finding the center value over a period of time.
  */
-class AnalogAccelerometer : public Sendable,
-                            public SendableHelper<AnalogAccelerometer> {
+class AnalogAccelerometer : public wpi::Sendable,
+                            public wpi::SendableHelper<AnalogAccelerometer> {
  public:
   /**
    * Create a new instance of an accelerometer.
@@ -93,7 +92,7 @@ class AnalogAccelerometer : public Sendable,
    */
   void SetZero(double zero);
 
-  void InitSendable(SendableBuilder& builder) override;
+  void InitSendable(wpi::SendableBuilder& builder) override;
 
  private:
   /**