ot::Spinel::Decoder::SavePosition() method

Saves the current read position in the frame. A subsequent call to `SavePosition()` will overwrite the previously saved position. The saved position can be used to move the read position back (using `ResetToSaved()`) and re-read the same content. Saved position can be within an open struct, and it remembers its enclosing struct. When the enclosing struct is closed, the saved position will be voided and can no longer be used. This ensures that we cannot jump back to middle an already fully decoded/read and closed struct.