Improve slicing and error handling around slices #724

Job	Run time
Lint - nightly toolchain	1m 51s
Docs - stable toolchain	1m 21s
Bench - nightly toolchain	1m 50s
Check WASM	25s
Test - msrv toolchain (recent)	1m 30s
Cross test	1m 43s
Test - stable toolchain (recent)	6m 12s
Test 32-bit version	46s
Docs - nightly toolchain	1m 13s
Test - nightly toolchain (recent)	6m 7s
	22m 58s

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