Make CreateKey faster

[bgrainger]

This addresses a "TODO" in the code:

• Use BinaryPrimitives instead of BinaryWriter to write directly to the array instead of via a MemoryStream.
• Move allocation of the key outside the loop to avoid creating unnecessary temporary objects.

For a local Minecraft map, this change reduced the first phase from 3:33.6 to 3:21.7.

[chrisl8]

Yeah, this repo seems abandoned.

If I learn how to put binaries up I could make temporary releases until the original author has time again.