Add bit operations

[dzmitry-lahoda]

.NET Core 3.0 got BitOperations

https://github.com/dotnet/corefx/blob/release/3.0/src/Common/src/CoreLib/System/Numerics/BitOperations.cs

Please add these into this lib for 128 bit number.

I can add these ops if you will five me right for repo.

I work on https://github.com/dzmitry-lahoda/NetStack/blob/master/Assets/Scripts/src/NetStack.Serialization/Coder.cs and need 91 bit number in one case (128 seems will be ok).

[dzmitry-lahoda]

Here are some

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static u128 WriteBit(u128 value, int bitOffset, bool on)
        {
            u128 onn = on ? u128.One << bitOffset : 0;
            return (value & ~(u128.One << bitOffset)) | onn;
        }

                [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static bool ExtractBit(u128 value, int bitOffset)
            => (value & (u128.One << bitOffset)) != 0;

I guess these are not optimal. I could add https://github.com/dotnet/BenchmarkDotNet then XUnit. And optimzia perf of this if you will give me rights onto the repo.