Add conversions into `UnsyncBoxBody` #1135

Job	Run time
cargo-hack	1m 22s
check	1m 2s
check-docs	47s
cargo-deny check (advisories)	21s
cargo-deny check (bans licenses sources)	20s
cargo-public-api-crates	47s
style	9s
test-msrv	2m 8s
test-versions (stable)	1m 29s
test-versions (beta)	1m 37s
test-versions (nightly)	1m 16s
	11m 18s

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