Improve slicing and error handling around slices #722

Job	Run time
Test - msrv toolchain (recent)	1m 37s
Bench - nightly toolchain	1m 56s
Check WASM	31s
Docs - stable toolchain	1m 18s
Test - stable toolchain (recent)	5m 58s
Cross test	1m 46s
Lint - nightly toolchain	1m 28s
Docs - nightly toolchain	1m 20s
Test - nightly toolchain (recent)	5m 44s
Test 32-bit version	49s
	22m 27s

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