Support Scala 2.13 standard library

[allanrenucci]

• Compile the library
• Compile and run the library tests
• Add it to the community build (and to our tests?)
• Upstream changes to scala/scala

[allanrenucci]

Here is a list of incompatibilities between the 2.12 and 2.13 that will make cross compiling (without warning) with the 2.12 and 2.13 std lib difficult:

• Stream is deprecated in favor of LazyList which does not exist in 2.12. Most usages can probably be replaced by a strict collection or an Iterator
• .zipped is deprecated in favor .lazyZip which does not exist in 2.12.
• On Map, mapValues is deprecated in favor of view.mapValues which does not work in 2.12
• mutable.SeqOps::transform is deprecated in favor of mapInPlace which does not exist in 2.12

Will update this comment when I find more.

[allanrenucci]

@sjrd since you're compiling Scala.js without warning on both 2.12 and 2.13, I would be happy to know if you had to workaround some of these issues

[sjrd]

• Don't use Stream nor LazyList. Use iterators.
• You can pimp .lazyZip on relevant collections. On 2.13 the "native" one will be used, and on 2.12 your pimp will be used.
• Use map instead of mapValues.
• Pimp mapInPlace on the old collections.