Emit a performance warning when OpenStruct is used #140

Job	Run time
build (3.2 / ubuntu-latest)	14s
build (3.2 / macos-latest)	20s
build (3.1 / ubuntu-latest)	9s
build (3.1 / macos-latest)	19s
build (3.0 / ubuntu-latest)	10s
build (3.0 / macos-latest)	15s
build (2.7 / ubuntu-latest)	32s
build (2.7 / macos-latest)	47s
build (2.6 / ubuntu-latest)	24s
build (2.6 / macos-latest)	1m 13s
build (2.5 / ubuntu-latest)	26s
build (2.5 / macos-latest)	1m 11s
build (head / ubuntu-latest)	9s
build (head / macos-latest)	20s
build (truffleruby-head / ubuntu-latest)	29s
build (truffleruby-head / macos-latest)	1m 11s
	8m 9s

Provide feedback