Calculate stop to subshape mapping during import #1087

Job	Run time
formatting	41s
msvc (Debug)	10m 58s
macos (macos-x86_64, On)	10m 8s
linux (linux-amd64-release, linux-amd64)	5m 28s
msvc (Release)	13m 8s
macos (macos-arm64)	9m 39s
linux (linux-arm64-release, linux-arm64, qemu-aarch64-static)	5m 41s
linux (clang-tidy)	12m 24s
linux (linux-sanitizer)	7m 43s
linux (linux-debug, valgrind --leak-check=full --error-exitcode=1)	5m 18s
	1h 21m 8s

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