[ntcore] Remove pImpl from implementation (#5480)

Also change Timestamped into a template.

ntcore/src/main/native/cpp/ValueCircularBuffer.cpp

@@ -0,0 +1,17 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "ValueCircularBuffer.h"
+
+using namespace nt;
+
+std::vector<Value> ValueCircularBuffer::ReadValue() {
+  std::vector<Value> rv;
+  rv.reserve(m_storage.size());
+  for (auto&& val : m_storage) {
+    rv.emplace_back(std::move(val));
+  }
+  m_storage.reset();
+  return rv;
+}