Backport slicing improvements 0.26.x #735

Job	Run time
Prepare	3s
Docs - stable toolchain (recent)	9s
Test - stable toolchain	7s
Check WASM	32s
Lint - nightly toolchain (recent)	47s
Test - nightly toolchain	17s
Test - MSRV	2m 50s
Docs - nightly toolchain (recent)	14s
	4m 59s

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