v1.0.0-preview6

This release adds UnsafePtr.AddByteOffset() and SubtractByteOffset(), and reworks UnsafePtr.AsPointer() to return pointers, not ref pointers. The latter returns a pointer with the same level of indirection as the input parameter, so ref T -> T* (same as Unsafe.AsPointer() but strongly-typed), ref T* -> T**, and ref T** -> T***.