FS.Archive: Clean up treatment of archives and their handles

- Refactor FS::Archive internals to make Archive creation and lifetime
  management clearer.
- Remove the "Archive as a File" hack.
- Implement 64-bit Archive handles.

src/core/file_sys/archive_backend.h

@@ -220,36 +220,6 @@ public:
      * @return Opened directory, or nullptr
      */
     virtual std::unique_ptr<DirectoryBackend> OpenDirectory(const Path& path) const = 0;
-
-    /**
-     * Read data from the archive
-     * @param offset Offset in bytes to start reading data from
-     * @param length Length in bytes of data to read from archive
-     * @param buffer Buffer to read data into
-     * @return Number of bytes read
-     */
-    virtual size_t Read(const u64 offset, const u32 length, u8* buffer) const = 0;
-
-    /**
-     * Write data to the archive
-     * @param offset Offset in bytes to start writing data to
-     * @param length Length in bytes of data to write to archive
-     * @param buffer Buffer to write data from
-     * @param flush  The flush parameters (0 == do not flush)
-     * @return Number of bytes written
-     */
-    virtual size_t Write(const u64 offset, const u32 length, const u32 flush, u8* buffer) = 0;
-
-    /**
-     * Get the size of the archive in bytes
-     * @return Size of the archive in bytes
-     */
-    virtual size_t GetSize() const = 0;
-
-    /**
-     * Set the size of the archive in bytes
-     */
-    virtual void SetSize(const u64 size) = 0;
 };
 
 } // namespace FileSys