`Interval` is so barebones compared to `LocalDateRange`

[someniatko]

I suggest Interval should have the following methods, like in LocalDateRange:

• contains(Instant): bool
• intersectsWith(Interval): bool
• getIntersectionWith(Interval): Interval
• of(Instant, Instant): self - for consistency with other classes

and nice to haves (also contained in the LocalDateRange):

• isEqualTo(Interval): bool
• toNativeDatePeriod(): \DatePeriod