chore(book): add simple calculator example #136

Job	Run time
Rustdoc	30s
Tests (1.74.0, macos-latest)	1m 6s
Tests (1.74.0, macos-latest, --features forbid_unsafe)	1m 7s
Tests (1.74.0, ubuntu-latest)	1m 10s
Tests (1.74.0, ubuntu-latest, --features forbid_unsafe)	1m 4s
Tests (1.74.0, windows-latest)	2m 22s
Tests (1.74.0, windows-latest, --features forbid_unsafe)	2m 20s
Tests (1.82.0, macos-latest)	1m 7s
Tests (1.82.0, macos-latest, --features forbid_unsafe)	1m 1s
Tests (1.82.0, ubuntu-latest)	1m 1s
Tests (1.82.0, ubuntu-latest, --features forbid_unsafe)	58s
Tests (1.82.0, windows-latest)	2m 33s
Tests (1.82.0, windows-latest, --features forbid_unsafe)	2m 30s
Tests (stable, macos-latest)	26s
Tests (stable, macos-latest, --features forbid_unsafe)	24s
Tests (stable, ubuntu-latest)	55s
Tests (stable, ubuntu-latest, --features forbid_unsafe)	1m 1s
Tests (stable, windows-latest)	2m 54s
Tests (stable, windows-latest, --features forbid_unsafe)	3m 30s
Tests (beta, macos-latest)	1m 37s
Tests (beta, macos-latest, --features forbid_unsafe)	1m 0s
Tests (beta, ubuntu-latest)	1m 0s
Tests (beta, ubuntu-latest, --features forbid_unsafe)	59s
Tests (beta, windows-latest)	2m 10s
Tests (beta, windows-latest, --features forbid_unsafe)	2m 35s
Tests (nightly, macos-latest)	58s
Tests (nightly, macos-latest, --features forbid_unsafe)	1m 3s
Tests (nightly, ubuntu-latest)	55s
Tests (nightly, ubuntu-latest, --features forbid_unsafe)	53s
Tests (nightly, windows-latest)	2m 22s
Tests (nightly, windows-latest, --features forbid_unsafe)	2m 19s
	45m 50s

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