Add Order#fromLessThan

[vlovgr]

I sometimes find myself writing Order instances using a 'less than' function. There is no such helper function for Order, but there is one for Ordering, which means that you currently have to rely on the fromOrdering function as follows.

import cats.Order
import java.time.Instant

implicit val orderInstant: Order[Instant] =
  Order.fromOrdering(Ordering.fromLessThan(_ isBefore _))

This pull request adds a fromLessThan function to Order, so we don't have to use Ordering.

implicit val orderInstant: Order[Instant] =
  Order.fromLessThan(_ isBefore _)

[codecov-io]

Codecov Report

Merging #2477 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2477      +/-   ##
==========================================
- Coverage   95.38%   95.35%   -0.03%     
==========================================
  Files         357      357              
  Lines        6517     6525       +8     
  Branches      279      281       +2     
==========================================
+ Hits         6216     6222       +6     
- Misses        301      303       +2

Impacted Files	Coverage Δ
kernel/src/main/scala/cats/kernel/Order.scala	90.9% <100%> (+2.02%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	96% <0%> (-4%)	⬇️
...rc/main/scala/cats/kernel/instances/function.scala	96.96% <0%> (-3.04%)	⬇️

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[kailuowang]

nice! thanks!