OPT: use a fast list buffer that isn't itself a collection.
OPT: override isActive is the CPS AnalyzerPlugin, rather than forwarding to the mixed in method.
OPT: avoid noop .seq call

We're in LinearSeqLike which extends TraversableOnce.
OPT: Cache run-specific definitions

Rather than looking them up and putting them in a Map
every implicit search.
OPT: Use mapList in RefChecks
OPT: Avoid varargs factory for List
OPT: directly use MurmurHash.listHash in List#hashCode
OPT: avoid Nil.equals in annotation checkers
OPT: map2 rather than Tuple2Zipped#map
OPT: cache ImplicitInfo#isCyclicOrErroneous
OPT: make isError bimorphic
OPT: tail recursive fold in AnalyzerPlugins.pluginsPt