implement rb_waitpid #879

Job	Run time
test (ubuntu-latest, 2.7, 1.61)	11m 33s
test (ubuntu-latest, 2.7, stable)	6m 11s
test (ubuntu-latest, 3.0, 1.61)	11m 40s
test (ubuntu-latest, 3.0, stable)	6m 26s
test (ubuntu-latest, 3.1, 1.61)	15m 24s
test (ubuntu-latest, 3.1, stable)	6m 6s
test (ubuntu-latest, 3.2, 1.61)	11m 43s
test (ubuntu-latest, 3.2, stable)	6m 52s
test (ubuntu-latest, head, 1.61)	12m 25s
test (ubuntu-latest, head, stable)	6m 6s
test (macos-latest, 2.7, 1.61)	13m 24s
test (macos-latest, 2.7, stable)	6m 49s
test (macos-latest, 3.0, 1.61)	13m 15s
test (macos-latest, 3.0, stable)	7m 31s
test (macos-latest, 3.1, 1.61)	13m 16s
test (macos-latest, 3.1, stable)	6m 27s
test (macos-latest, 3.2, 1.61)	13m 37s
test (macos-latest, 3.2, stable)	6m 32s
test (macos-latest, head, 1.61)	8m 41s
test (macos-latest, head, stable)	5m 4s
test (windows-latest, 2.7, 1.61)	9m 1s
test (windows-latest, 2.7, stable)	5m 26s
test (windows-latest, 3.0, 1.61)	8m 0s
test (windows-latest, 3.0, stable)	4m 23s
test (windows-latest, 3.1, 1.61)	9m 33s
test (windows-latest, 3.1, stable)	4m 26s
test (windows-latest, 3.2, 1.61)	10m 4s
test (windows-latest, 3.2, stable)	5m 0s
test (windows-latest, mswin, stable)	3m 48s
	4h 8m 43s

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