Calculate stop to subshape mapping during import #1073

Job	Run time
formatting	46s
macos (macos-x86_64, On)	10m 58s
linux (linux-amd64-release, linux-amd64)	29s
msvc (Debug)	8m 3s
linux (linux-arm64-release, linux-arm64, qemu-aarch64-static)	34s
msvc (Release)	9m 16s
macos (macos-arm64)	19m 56s
linux (clang-tidy)	8m 16s
linux (linux-sanitizer)	4m 21s
linux (linux-debug, valgrind --leak-check=full --error-exitcode=1)	3m 50s
	1h 6m 29s

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