consensus: implement `root_slow` for `Receipts` #4769

Job	Run time
check no_std	1m 0s
cfg-check	38s
clippy	1m 30s
docs	1m 11s
doctest	2m 44s
feature-checks	8m 44s
fmt	13s
test ubuntu-latest stable --no-default-features	2m 17s
wasm-unknown	2m 20s
wasm-wasi	3m 13s
test ubuntu-latest stable	2m 28s
test ubuntu-latest stable --all-features	3m 6s
test ubuntu-latest nightly --no-default-features	1m 33s
test ubuntu-latest nightly	1m 34s
test ubuntu-latest nightly --all-features	2m 14s
test ubuntu-latest 1.79 --no-default-features	1m 28s
test ubuntu-latest 1.79	1m 29s
test windows-latest stable --no-default-features	5m 56s
test windows-latest stable	6m 5s
test windows-latest stable --all-features	5m 59s
test windows-latest nightly --no-default-features	4m 17s
test windows-latest nightly	5m 10s
test windows-latest nightly --all-features	5m 15s
test windows-latest 1.79 --no-default-features	3m 14s
test windows-latest 1.79	3m 6s
	1h 16m 44s

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