Fix issue #680 with encoding int-keyed maps. #601

Job	Run time
Test in i386 (windows-latest, 1.21)	3m 54s
Test in i386 (ubuntu-latest, 1.22)	2m 35s
Test in i386 (windows-latest, 1.22)	4m 1s
Test in i386 (windows-latest, 1.23)	3m 55s
page	44s
lint	15s
Test in i386 (ubuntu-latest, 1.21)	2m 29s
Test in i386 (ubuntu-latest, 1.23)	24s
Test with -race (macos-latest, 1.21)	2m 38s
Coverage	22s
Test with -race (macos-latest, 1.22)	2m 37s
Test with -race (ubuntu-latest, 1.23)	27s
Test with -race (ubuntu-latest, 1.22)	27s
Test with -race (windows-latest, 1.21)	1m 54s
ycat	2m 21s
Test with -race (ubuntu-latest, 1.21)	23s
Test with -race (macos-latest, 1.23)	2m 39s
Test with -race (windows-latest, 1.23)	1m 56s
Test with -race (windows-latest, 1.22)	1m 53s
Fuzzing Test	1m 15s
	37m 9s

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