Interval comparison is weird

[DifferentialOrange]

Bug description

Datetime interval type comparison yield counter-intuitive results.

• OS: Linux
• OS Version: Ubuntu 20.04
• Architecture: amd64

Tarantool 2.10.1-0-g482d91c66

Steps to reproduce

tarantool> datetime.interval.new{hour=1} == datetime.interval.new{hour=1}
---
- true
...

tarantool> datetime.interval.new{hour=1} == datetime.interval.new{min=60}
---
- false
...

tarantool> datetime.interval.new{hour=1} > datetime.interval.new{min=60}
---
- true
...

tarantool> datetime.interval.new{hour=1} < datetime.interval.new{min=60}
---
- false
...

tarantool> datetime.interval.new{hour=1} < datetime.interval.new{min=61}
---
- false
...

Expected behavior

Simple approach to == comparison is rather understandable, but >, <, >=, <= is really confusing. @ImeevMA said that intervals are not designed to be comparable by nature, so maybe it's worth to forbid using compare operators in Lua.

[tsafin]

It behaves so by design, as per RFC. @Mons what do you think?

[DifferentialOrange]

It behaves so by design

But the design contradicts basic real numbers arithmetic (we use the same adjust rules and non-ambiguous units like hours).

tarantool> x = datetime.new()
---
...

tarantool> y = datetime.interval.new{hour=1}
---
...

tarantool> z = datetime.interval.new{min=61}
---
...

tarantool> y > z
---
- true
...

tarantool> x + y > x + z
---
- false
...

[tsafin]

@Mons looks, for interval objects, like we should only overload equality operators, and not try to be so much helpful and overload comparisons. Because they contradict usual expectations.