Don't use Vector<byte>.Dot

[benaadams]

Vector<byte>.Dot is not an intrinsic so quite slow; see https://github.com/dotnet/coreclr/issues/2359 so don't use it.

Vector.Dot<byte> is not an intrinsic single CPU instruction and is instead composed of a new object create followed by 32 byte multiply instructions and 32 byte addition instructions (depending on arch).

For the aspnet5 server Kestrel this adds up to being 8.23% of the total server CPU load as it makes heavy use of it when parsing the ASCII headers.

Need alternative implementation for MemoryPoolIterator2.Seek