add Chapter04 bare example

Chapter04/bare/hal/uart/inc/uart_stm32.hpp

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <cstdint>
+
+#include <uart.hpp>
+
+#include <stm32f0xx_hal.h>
+
+namespace hal
+{
+class uart_stm32 : public uart
+{
+  public:
+    uart_stm32(USART_TypeDef *inst, std::uint32_t baud = 115200);
+
+    void init() override;
+
+    void putchar(char c) override;
+
+  private:
+    UART_HandleTypeDef huart;
+    USART_TypeDef *instance;
+    std::uint32_t baudrate;
+};
+}; // namespace hal