put the compare perf script into the repo (#564)
put the compare perf script into the repo (#564)

Motivation:

For a long while I had a script which runs NIO's performance tests
against different Swift toolchains. It should be in the repository.

Modifications:

add scripts/compare_perf_of_swift_versions.sh

Result:

everybody can run the performance tests against different Swift versions
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio