Improve slicing and error handling around slices #721

Job	Run time
Bench - nightly toolchain	2m 2s
Docs - stable toolchain	1m 19s
Cross test	1m 40s
Test 32-bit version	40s
Test - nightly toolchain (recent)	5m 45s
Check WASM	35s
Test - stable toolchain (recent)	5m 51s
Test - msrv toolchain (recent)	1m 28s
Lint - nightly toolchain	1m 39s
Docs - nightly toolchain	1m 15s
	22m 14s

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