Support long collections

[jfheins]

Hi,
I tried to use your nuget package as I was looking for a fast and compact serializer & deserializer. When I tried to serialize a large 2D array of ushort, it did not work properly.

The reason is that I want to serialize 5 million values and as a 2D array is not supported, I put them into a 1D array.
But then the length seems to be limited to int16.

It would be nice to annotate (potentially) long collections to have a 4 byte (or even 8 byte) length field.