Verify Fp2ElementRef implementation

[romanstrobl]

Some of the methods in Fp2Element class differ from the official implementation (e.g. sqrt). Although these methods are well tested, we should compare the performance and security parameters with the official implementation.

[romanstrobl]

This issue has low priority because the sqrt method is used only in the reference implementation, which is not likely to be used for other than testing purposes.

[petrdvorak]

Not required anymore.