Optimization for non-nullable reference types #14

Don't emit a null byte marker for non-nullable reference types

Based on metadata described in https://github.com/dotnet/roslyn/blob/master/docs/features/nullable-metadata.md

Arrays are problematic for this, because while they offer the most potential benefit, it's not uncommon to have T[] where some elements are actually null (reserved space, sparse array, etc).