Optimize EmptySet

[retronym]

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[dwijnand]

Sounds too good to be true... does it pass mima?

[retronym]

@dwijnand Sadly no.

[error] scala-library: Failed binary compatibility check against org.scala-lang:scala-library:2.12.13! Found 2 potential problems (filtered 74)
[error]  * method intersect(scala.collection.GenSet)scala.collection.immutable.Set in object scala.collection.immutable.Set#EmptySet does not have a correspondent in other version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.immutable.Set#EmptySet.intersect")
[error]  * method diff(scala.collection.GenSet)scala.collection.immutable.Set in object scala.collection.immutable.Set#EmptySet does not have a correspondent in other version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.immutable.Set#EmptySet.diff")

Could still pull this up into a special case in the base class implementation.