Rust

Add more tests for send and sync behavior, support unsized types #335

Job	Run time
valgrind	2m 21s
fmt	16s
build (nightly, --all-features)	39s
build (beta, --features collections,boxed)	41s
benches	25s
build (stable, --features collections,boxed)	34s
miri	4m 7s
build (1.73.0, --features collections,boxed)	51s
build (stable, --no-default-features)	43s
	10m 37s

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