[Feature #21781] Add `ENV.fetch_values` #2348

Job	Run time
RuboCop	52s
specs (ubuntu, 3.4.8)	1m 43s
specs (ubuntu, 3.3.10)	1m 30s
specs (windows, 3.4.8)	3m 38s
specs (ubuntu, 4.0.1, --repeat 2)	2m 23s
specs (ubuntu, 4.0.1, --parser=parse.y)	1m 38s
specs (macos, 4.0.1)	1m 44s
specs (ubuntu, 4.0.1, LC_ALL=C)	1m 28s
specs (ubuntu, 4.0.1)	1m 37s
specs (windows, 4.0.1)	4m 9s
specs (ubuntu, 4.0.1, --enable-frozen-string-literal)	1m 40s
specs (macos, 3.3.10)	1m 43s
specs (windows, 3.3.10)	3m 19s
specs (macos, 3.4.8)	1m 34s
	28m 58s

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