Backport slicing improvements 0.26.x #733

Job	Run time
Docs - stable toolchain (recent)	41s
Check WASM	29s
Test - stable toolchain	2m 37s
Prepare	5s
Docs - nightly toolchain (recent)	42s
Lint - nightly toolchain (recent)	36s
Test - MSRV	6m 45s
Test - nightly toolchain	2m 39s
	14m 34s

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