Add new file IO callback API.

[Clownacy]

This new one avoids the need for memory allocation, streamlines some library internals, and provides a simpler interface for the user:

Old API:

core_file* const file_callbacks = (core_file*)malloc(sizeof(core_file));

if (file_callbacks == NULL)
{
	return NULL;
}
else
{
	chd_file *file;

	file_callbacks->argp = &disc->file;
	file_callbacks->fsize = ClownCD_Disc_CHDFileSize;
	file_callbacks->fread = ClownCD_Disc_CHDFileRead;
	file_callbacks->fclose = ClownCD_Disc_CHDFileClose;
	file_callbacks->fseek = ClownCD_Disc_CHDFileSeek;

	chd_open_core_file(file_callbacks, CHD_OPEN_READ, NULL, &file);
	return file;
}

New API:

static const core_file_callbacks file_callbacks = {
	ClownCD_Disc_CHDFileSize,
	ClownCD_Disc_CHDFileRead,
	ClownCD_Disc_CHDFileClose,
	ClownCD_Disc_CHDFileSeek,
};

chd_file *file;

chd_open_core_file_callbacks(&file_callbacks, &disc->file, CHD_OPEN_READ, NULL, &file);
return file;

The old API has been preserved for backwards-compatibility; since libchdr is packaged by Debian, I figured that API-breaking changes would be undesirable.

I also threw-in some miscellaneous changes that I had lying around; they can be removed if you want them submitting separately.

[Clownacy]

Closing since #143 makes this redundant.