Improve slicing and error handling around slices #723

Job	Run time
Test 32-bit version	59s
Cross test	1m 34s
Check WASM	34s
Bench - nightly toolchain	1m 29s
Docs - stable toolchain	1m 15s
Test - msrv toolchain (recent)	1m 40s
Test - stable toolchain (recent)	5m 33s
Docs - nightly toolchain	1m 26s
Lint - nightly toolchain	1m 25s
Test - nightly toolchain (recent)	5m 51s
	21m 46s

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