Improve type signature for Kernel.exec #3172

Job	Run time
windows (windows-2019, ucrt)	1m 12s
windows (windows-2019, mswin)	1m 15s
windows (windows-2022, ucrt)	1m 23s
windows (windows-2022, mswin)	2m 9s
test (master-nightly-focal, test)	2m 5s
test (master-nightly-focal, stdlib_test)	2m 9s
test (master-nightly-focal, rubocop validate test_doc build test_generate_stdlib)	2m 20s
test (master-nightly-focal, lexer confirm_lexer)	4m 38s
test (master-nightly-focal, lexer compile)	4m 48s
test (3.2-dev-focal, test)	2m 4s
test (3.2-dev-focal, stdlib_test)	2m 46s
test (3.2-dev-focal, rubocop validate test_doc build test_generate_stdlib)	3m 4s
test (3.2-dev-focal, lexer confirm_lexer)	3m 56s
test (3.2-dev-focal, lexer compile)	3m 59s
test (3.1-dev-focal, test)	2m 38s
test (3.1-dev-focal, rubocop validate test_doc build test_generate_stdlib)	3m 20s
test (3.1-dev-focal, lexer confirm_lexer)	4m 4s
test (3.1-dev-focal, lexer compile)	4m 55s
test (3.0-dev-focal, test)	2m 53s
test (3.0-dev-focal, rubocop validate test_doc build test_generate_stdlib)	2m 55s
test (3.0-dev-focal, lexer confirm_lexer)	5m 9s
test (3.0-dev-focal, lexer compile)	4m 45s
test (2.7-dev-bionic, test)	2m 11s
test (2.7-dev-bionic, rubocop validate test_doc build test_generate_stdlib)	3m 8s
test (2.7-dev-bionic, lexer confirm_lexer)	3m 33s
test (2.7-dev-bionic, lexer compile)	3m 34s
test (2.6-bionic, test)	1m 54s
test (2.6-bionic, rubocop validate test_doc build test_generate_stdlib)	2m 10s
test (2.6-bionic, lexer confirm_lexer)	3m 57s
test (2.6-bionic, lexer compile)	3m 25s
	1h 32m 19s

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