Support for `rb_io_mode_t`. #391

Job	Run time
ruby-versions / Generate Ruby versions	2s
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	3m 40s
windows-latest 2.7	3m 40s
windows-latest 3.0	1m 40s
windows-latest 3.0	1m 40s
windows-latest 3.1	1m 36s
windows-latest 3.1	1m 36s
windows-latest 3.2	1m 21s
windows-latest 3.2	1m 21s
windows-latest 3.3	2m 1s
windows-latest 3.3	2m 1s
windows-latest 3.4	1m 54s
windows-latest 3.4	1m 54s
windows-latest jruby-head	1m 26s
windows-latest jruby-head	2m 7s
windows-latest mingw	1m 24s
windows-latest mingw	1m 24s
windows-latest mswin	1m 47s
windows-latest mswin	1m 47s
	34m 23s

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