Add `SharedBackend` implementation #5

Job	Run time
doctest	30s
clippy	46s
cfg-check	32s
docs	30s
feature-checks	18s
fmt	11s
no-std	13s
test stable --no-default-features	45s
test stable	46s
test stable --all-features	42s
test nightly --no-default-features	42s
test nightly	35s
test nightly --all-features	34s
test 1.76 --no-default-features	31s
test 1.76	27s
	8m 2s

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