.NET7：更细致的时间

　　当年在做go时，很羡慕它的时间有微秒，纳秒，在做性能优化时，能很小颗粒度的查看引入方法执行的时间，当时.net的DateTime只有毫秒（虽然也有别的办法获取）。现在，在最新的.NET7 Preview4中，DateTime也有微秒和纳秒了，倍感亲切。

　　 纳秒在百位上，没有十位和个位，但这也说明.NET在进化，向高性能进化，在乎微秒和百位纳秒了（哈哈）。
　　下面是引入这两个时间单位的实现：

namespace System {
    public struct DateTime {
        public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int microsecond);
        public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int microsecond, System.DateTimeKind kind);
        public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, int microsecond, System.Globalization.Calendar calendar);
        public int Microsecond { get; }
        public int Nanosecond { get; }
        public DateTime AddMicroseconds(double value);
    }
    public struct DateTimeOffset {
        public DateTimeOffset(int year, int month, int day, int hour, int minute, int second, int millisecond, int microsecond, System.TimeSpan offset);
        public DateTimeOffset(int year, int month, int day, int hour, int minute, int second, int millisecond, int microsecond, System.TimeSpan offset, System.Globalization.Calendar calendar);
        public int Microsecond { get; }
        public int Nanosecond { get; }
        public DateTimeOffset AddMicroseconds(double microseconds);
    }
    public struct TimeSpan {
        public const long TicksPerMicrosecond = 10L;
        public const long NanosecondsPerTick = 100L;
        public TimeSpan(int days, int hours, int minutes, int seconds, int milliseconds, int microseconds);
        public int Microseconds { get; }
        public int Nanoseconds { get; }
        public double TotalMicroseconds { get; }
        public double TotalNanoseconds { get; }
        public static TimeSpan FromMicroseconds(double microseconds);
    }
    public struct TimeOnly {
        public TimeOnly(int day, int hour, int minute, int second, int millisecond, int microsecond);
        public int Microsecond { get; }
        public int Nanosecond { get; }