feat: add map_inner #1627

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Job	Run time
doctest	1m 10s
test stable --no-default-features	1m 11s
test stable	1m 6s
test stable --all-features	1m 43s
test nightly --no-default-features	1m 14s
test nightly	1m 12s
no-std	19s
test nightly --all-features	1m 51s
test 1.76 --no-default-features	36s
wasm-unknown	38s
test 1.76	41s
wasm-wasi	1m 22s
clippy	42s
docs	55s
feature-checks	2m 4s
cfg-check	29s
fmt	11s
	17m 24s

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