Reflection: mirrors can be made faster

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This bug is essentially about submitting a pull request from https://github.com/scalamacros/kepler/tree/zzz/reflection-opt3

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Imported From: https://issues.scala-lang.org/browse/SI-6638?orig=1
Reporter: @xeno-by

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@xeno-by said:
scala/scala#1821

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@xeno-by said:
Fixed in scala/scala@186e3bf