consensus: implement `root_slow` for `Receipts` #4768

Job	Run time
wasm-unknown	41s
test ubuntu-latest stable --no-default-features	51s
clippy	44s
test ubuntu-latest stable	53s
check no_std	26s
docs	28s
test ubuntu-latest stable --all-features	1m 12s
doctest	46s
feature-checks	35s
test ubuntu-latest nightly --no-default-features	38s
wasm-wasi	39s
test ubuntu-latest nightly	31s
test ubuntu-latest nightly --all-features	57s
cfg-check	27s
test ubuntu-latest 1.79 --no-default-features	43s
fmt	12s
test ubuntu-latest 1.79	51s
test windows-latest stable --no-default-features	2m 57s
test windows-latest stable	3m 34s
test windows-latest stable --all-features	3m 14s
test windows-latest nightly --no-default-features	2m 38s
test windows-latest nightly	1m 45s
test windows-latest nightly --all-features	2m 40s
test windows-latest 1.79 --no-default-features	2m 20s
test windows-latest 1.79	1m 57s
	32m 39s

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