fix: Correctly calculate highest_field_id in schema (#590) #5

Job	Run time
unit	7m 42s
check (ubuntu-latest)	5m 21s
build_with_no_default_features (ubuntu-latest)	1m 32s
build (ubuntu-latest)	4m 37s
check (macos-latest)	5m 30s
build_with_no_default_features (macos-latest)	1m 22s
build (macos-latest)	3m 47s
build_with_no_default_features (windows-latest)	3m 49s
build (windows-latest)	12m 11s
	45m 51s

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