Avoid allocations in DateTime.Now

Avoid allocations in DateTime.Now

Every call to DateTime.Now ends up allocating a DaylightTime as it computes the offset from UtcNow.  DaylightTime is just a plain old object that stores two DateTimes and a Timespan.  Ideally we could just change it to be a struct, but it's public.  Instead, this commit adds an internal DaylightTimeStruct, which is the same as DaylightTime except as a value type, and changes some of the internal uses of DaylightTime to instead use DaylightTime.  As a result, DateTime.Now is allocation-free.