File utils needed

[wisonye]

SmartFile is an opaque pointer to the following struct:

struct File {
    inner: FILE
}

The interface should look like this:

• SMART_FILE macro to wrap FILE *, or just use MAKE_UNIQUE_PTR macro?

• String FU_read_to_string(SmartFile file)

• Vec FU_read_lines(SmartFile file)

• usize FU_write_to_file(SmartFile file, u8 *bytes, usize len)

• FU_append_to_file(SmartFile file, u8 *bytes, usize len)

• FU_delete_file(SmartFile file)

• FU_copy_file(SmartFile file, const char *new_filename)

• FU_move_file(SmartFile file, const char *new_filename)