Add `SharedBackend` implementation #7

Job	Run time
docs	25s
cfg-check	24s
doctest	37s
clippy	26s
feature-checks	36s
fmt	18s
no-std	14s
test stable --no-default-features	32s
test stable	25s
test stable --all-features	35s
test nightly --no-default-features	38s
test nightly	38s
test nightly --all-features	34s
test 1.76 --no-default-features	34s
test 1.76	32s
	7m 28s

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