The first implementation of a set of new I/O APIs.

The intention of this change includes basic support of data reference,
segment and supplying of a custom I/O interface.
Define a data type to handle a file.

At present, this is equivalent to lsmash_root_t.
After updating to ver. 2.0.0, this will be an incomplete data type in the
public header.
io: Multiple file handling in a ROOT.