Remove OpenStruct #352

Job	Run time
lint	14s
test (3.3, 7.2, --enable-frozen-string-literal)	1m 10s
test (3.3, 7.1, --enable-frozen-string-literal)	1m 13s
test (3.3, 7.0, --enable-frozen-string-literal)	1m 17s
test (3.3, 6.1, --enable-frozen-string-literal)	1m 10s
test (3.2, 7.2)	1m 10s
test (3.2, 7.1)	1m 6s
test (3.2, 7.0)	1m 10s
test (3.2, 6.1)	1m 8s
test (3.1, 7.1)	1m 6s
test (3.1, 7.0)	1m 5s
test (3.1, 6.1)	1m 5s
test (3.0, 7.0)	1m 4s
test (3.0, 6.1)	1m 2s
test (jruby-9.4.5.0, 7.0)	3m 53s
test (2.7, 7.0)	1m 18s
test (2.7, 6.1)	1m 18s
test (2.7, 6.0)	1m 22s
test (jruby-9.3.10.0, 6.1)	3m 30s
test (2.6, 5.2)	1m 13s
	27m 34s

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