Serialize to file

[SashaOv]

com.kamikaze.docidset.impl.PForDeltaDocIdSet#serialize and com.kamikaze.docidset.impl.PForDeltaDocIdSet#deserialize write to and read from memory buffer. I am using big datasets and I think I cannot afford spending twice as much memory, so I will have to use EWAHCompressedBitmap for now.

I would like to be able to write to file/stream directly as in:

public void serialize(OutputStream out)
public void deserialize(InputStream in)