[ntcore] Remove pImpl from implementation (#5480)

Also change Timestamped into a template.

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

@@ -0,0 +1,49 @@
+// 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.
+
+#pragma once
+
+#include <utility>
+#include <vector>
+
+#include <wpi/circular_buffer.h>
+
+#include "Value_internal.h"
+#include "networktables/NetworkTableValue.h"
+#include "ntcore_cpp_types.h"
+
+namespace nt {
+
+class ValueCircularBuffer {
+ public:
+  explicit ValueCircularBuffer(size_t size) : m_storage{size} {}
+
+  template <class... Args>
+  void emplace_back(Args&&... args) {
+    m_storage.emplace_back(std::forward<Args...>(args...));
+  }
+
+  std::vector<Value> ReadValue();
+  template <ValidType T>
+  std::vector<Timestamped<typename TypeInfo<T>::Value>> Read();
+
+ private:
+  wpi::circular_buffer<Value> m_storage;
+};
+
+template <ValidType T>
+std::vector<Timestamped<typename TypeInfo<T>::Value>>
+ValueCircularBuffer::Read() {
+  std::vector<Timestamped<typename TypeInfo<T>::Value>> rv;
+  rv.reserve(m_storage.size());
+  for (auto&& val : m_storage) {
+    if (IsNumericConvertibleTo<T>(val) || IsType<T>(val)) {
+      rv.emplace_back(GetTimestamped<T, true>(val));
+    }
+  }
+  m_storage.reset();
+  return rv;
+}
+
+}  // namespace nt