Cross compile replace Stream with LazyList on Scala 2.13

[kailuowang]

• Added LazyList as a type alias for Stream on Scala 2.12- to support cross compilation
• Thanks to backward compatibility, most existing class/trait names remained as StreamXXXX.
• Added instances for Scala2.13 LazyList (all the credit goes to @johnynek )

[codecov-io]

Codecov Report

Merging #2904 into master will decrease coverage by 0.02%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##           master    #2904      +/-   ##
==========================================
- Coverage    94.2%   94.17%   -0.03%     
==========================================
  Files         368      370       +2     
  Lines        6949     6952       +3     
  Branches      309      307       -2     
==========================================
+ Hits         6546     6547       +1     
- Misses        403      405       +2

Impacted Files	Coverage Δ
...main/scala-2.12-/cats/kernel/compat/lazyList.scala	0% <0%> (ø)
...re/src/main/scala-2.12-/cats/compat/lazyList.scala	0% <0%> (ø)
.../scala/cats/kernel/instances/StreamInstances.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/instances/sortedSet.scala	100% <100%> (ø)	⬆️
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	99.15% <100%> (ø)	⬆️
...e/src/main/scala-2.12-/cats/instances/stream.scala	97.26% <100%> (ø)
core/src/main/scala/cats/data/package.scala	88.88% <100%> (ø)	⬆️
core/src/main/scala/cats/instances/parallel.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/OneAnd.scala	95.94% <100%> (ø)	⬆️
core/src/main/scala/cats/data/ZipStream.scala	77.77% <50%> (ø)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d822620...9f6fff6. Read the comment docs.

[LukaJCB]

This is great, thanks!

[johnynek]

Thanks for doing this!