Add `IO::Error#target` #1297

Job	Run time
Build interpreter	14m 35s
Test Interpreter	5m 23s
Test std_spec with interpreter (0)	5m 9s
Test std_spec with interpreter (1)	3m 36s
Test std_spec with interpreter (2)	2m 54s
Test std_spec with interpreter (3)	2m 29s
	34m 6s

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