Add support for bigstring. #9548

Job	Run time
build_details	4s
pre-commit	3m 7s
build_js	1m 24s
tree_sitter_parse	56s
build_no_depopts	5m 3s
update_doc	0s
lezer_parse	51s
build_opam (4.14.x)	4m 56s
build_win32 (x64)	9m 36s
build_opam (5.0.x)	4m 47s
run_tests (@citest)	3m 27s
run_tests (@doctest)	3m 26s
build_posix (debian_trixie, amd64)	6m 13s
build_posix (debian_bookworm, amd64)	5m 21s
run_tests (@mediatest)	3m 9s
build_posix (ubuntu_jammy, amd64)	5m 22s
build_posix (ubuntu_lunar, amd64)	6m 8s
build_posix (alpine, amd64)	5m 11s
fetch_s3_artifacts	0s
build_docker_alpine	0s
build_docker	0s
build_docker_minimal	0s
build_docker_alpine_minimal	0s
build_docker_release	0s
build_docker_release-minimal	0s
update_release	1s
	1h 9m 2s

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