wrong Double to UInt256 conversion

[Abalioha]

Issue can be easily tested with this snippet:

double d = 1E+20d;
UInt256 u = (UInt256)d; 
Console.WriteLine(u);   // 0

[Abalioha]

The problem goes deeper to
Lsh(in UInt256 x, int n, out UInt256 res)
when in and out params are the same

        var u = new UInt256(1);
        UInt256.Lsh(in u, 1, out u);
        Console.WriteLine(u);   // 0

[benaadams]

Resolved by #44 and #43 with full tests for all parameters