Remove allocations from Marvin.GenerateSeed

[ForNeVeR]

This removes some unnecessary allocations from Marvin.

[ForNeVeR]

The build error is:

10:52:26 /mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/Common/src/System/Marvin.cs(114,30):
error CS1503: Argument 1: cannot convert from 'System.Span<byte>' to 'byte[]'
[/mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/Common/tests/Performance/Common.Performance.Tests.csproj]

So, it doesn't know about RandomNumberGenerator.GetBytes(Span<byte>) (added in 2.1 IIRC).

I don't understand how to fix that. Are some performance tests still using old .NET Core version? It it intentional, so my changes cannot be merged? Or should I add a conditional compilation stuff to fix the code for netcoreapp2.0? If so, it seems too much for such a small benefit, so I should probably close the PR.

[jkotas]

This is not portable to platforms with strong alignment requirements. This should be MemoryMarshal.Read<ulong>(bytes) instead.

[jkotas]

Are some performance tests still using old .NET Core version?

It is not just the performance tests project. The same problem is in other projects that include this file and build against netstandard2.0.

It it intentional, so my changes cannot be merged?

The change would need to be conditional to make it build in all contexts, e.g.

#if netstandard
<original code>
#else
<you new code
#endif

[ForNeVeR]

Kindly thanks for your review. I think that conditional compilation is too much for such a small benefit (for single static property evaluation only), so I'll close that PR for now.