Use Buffer.MemoryCopy

[Anderman]

Buffer.BlockCopy has a large overhead for small size copy. This is because it has to find out a lot of things before it start copy see blockcopy.

MemoryCopy is almost a direct call to memmove see here

[benaadams]

Situation may have changed dotnet/coreclr#3118

[cesarblum]

Indeed. I don't think it's worth it making the switch to MemoryCopy. @halter73 what do you think?

[halter73]

Yeah. This clearly seems fixed. If there is any evidence we should still switch, I'm open to reopening.