add Chapter12

Chapter12/cpp_hal/hal/uart/inc/uart_stm32.hpp

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+#pragma once
+
+#include <span>
+
+#include <cstdint>
+
+#include <uart.hpp>
+
+#include <stm32f0xx_hal.h>
+#include <stm32f072xb.h>
+
+namespace hal
+{
+class uart_stm32 : public uart
+{
+  public:
+    uart_stm32(USART_TypeDef *inst);
+
+    void init(std::uint32_t baudrate = c_baudrate_default) override;
+
+    void write_array(const char * ptr, std::size_t len) override;
+
+  private:
+    UART_HandleTypeDef huart_;
+    USART_TypeDef *instance_;
+    std::uint32_t baudrate_;
+    static constexpr std::uint32_t c_baudrate_default = 115200;
+};
+}; // namespace hal