fix: RAII support for File class

When a File object goes out of scope, its resources are not freed. This
patch:

- Adds a destructor to free this memory and close the file handle when
  the object is destroyed.
- Deletes the copy constructor and copy assignment operator, as this
  class is not copyable (at least, not without more significant future
  work).
- Adds move semantics.

---

Fixes #66.

src/SD.cpp

@@ -243,6 +243,37 @@ File::File(FRESULT result /* = FR_OK */)
   _res = result;
 }
 
+File::File(File &&other)
+{
+  _name = other._name;
+  _fil = other._fil;
+  _dir = other._dir;
+  _res = other._res;
+  other._name = NULL;
+  other._fil = NULL;
+  other._dir = {};
+}
+
+File &File::operator=(File &&other)
+{
+  close();
+
+  _name = other._name;
+  _fil = other._fil;
+  _dir = other._dir;
+  _res = other._res;
+  other._name = NULL;
+  other._fil = NULL;
+  other._dir = {};
+
+  return *this;
+}
+
+File::~File()
+{
+  close();
+}
+
 /** List directory contents to Serial.
  *
  * \param[in] flags The inclusive OR of

src/STM32SD.h

@@ -33,6 +33,15 @@ uint8_t const LS_R = 4;
 class File {
   public:
     File(FRESULT res = FR_OK);
+
+    File(const File &) = delete;
+    File &operator=(const File &) = delete;
+
+    File(File &&other);
+    File &operator=(File &&other);
+
+    virtual ~File();
+
     virtual size_t write(uint8_t);
     virtual size_t write(const uint8_t *buf, size_t size);
     virtual size_t write(const char *buf, size_t size);