datetime: implement interval comparison

We used to not implement comparison operators for interval
objects, thus any compare (even of equal) objects returned false:

	```
	tarantool> dt1 = datetime.new()
	---
	...

	tarantool> dt2 = datetime.now()
	---
	...

	tarantool> dt1 - dt1 == dt2 - dt2
	---
	- false
	...
	```

Now we implemented comparison for interval objects, where we compare
field by field starting from highest attribute (e.g. `year`) to
smallest attribute (e.g. `nsec`) and if any of comparison
returned non zero value then we return this result.

NO_DOC=bugfix
NO_CHANGELOG=bugfix

(cherry picked from commit 65a3c17f)