PRs skipped when branch merging for 2.13.4

[dwijnand]

branch merge PR: scala/scala#9206

skipped PRs:

• Emit method reference lambdas without helper method scala#9022 Fix -Ydelambdafy:method-ref on a Java annotation scala#9178 -Ydelamdify:method-ref not worth it
• Map ++ without CBF should use the same optimised path as ++ with CBF scala#8986 Map.++ optimizations
• Small optimisations - some faster array copies, reuse empty arrays scala#9091 reuse empty array instances hard cherry-pick (Array and BitSet things have changed in 2.13)
• issue 12062 - don't box primitives for some simple cases [ci: last-only] scala#9102 don't box double.isNaN -> Don't box some simple primitives (forward-port) scala#9261

[NthPortal]

Map.++ optimisations are unlikely to be portable to 2.13

[dwijnand]

And again scala/scala#9231, skipping:

• avoid some LazyVals, prefer AbstractFunction scala#9216 avoid some LazyVals, prefer AbstractFunction hard cherry-pick (HashMap/HashSet and even MurmurHash3 have changed in 2.13)
• ListMap is reverse oriented, so use that for foldRight scala#9218 ListMap is reverse oriented, so use that for foldRight -> ListMap is reverse oriented, so use that for foldRight (forward-port) scala#9262
• ListMap.last should not generate additional garbage scala#9214 ListMap.last should not generate additional garbage hard: IndexedSeqLike and LinearSeqLike have been reworked

Lukas mentioned that ListMap is still a reversed linked list internally, so perhaps those ListMap changes are recoverable.

[dwijnand]

Spun out 2 PRs, the rest I've re-skipped.