Bug [rlwe]: rlwe.Parameters.ReadFrom fails (PR #456)

[Pro7ech]

#456 added WriteTo and ReadFrom to the rlwe.Parameters without adding associated tests and it actually fails when calling .ReadFrom.
This can be easily reproduced by testing the serialization of rlwe.Parameters with the method buffer.RequireSerializerCorrect, which returns the error: unexpected end of JSON input.

This happens because .BinarySize() is called on empty parameters by ReadFrom to get the number of bytes to read. But this returns the wrong size because the serialization size of rlwe.Parameters is dependant on the content of the struct.

-> Do like other content-dependent structs (e.g. ring.Poly): add a 4/8 bytes prefix with the number of bytes to read