Add a poisonable `Barrier` to `picos_std.sync` #4704

Job	Run time
test-on-windows (ocaml.5.2.1,ocaml-option-mingw)	13m 35s
build-on-lower-bound (ubuntu-latest, 4.12.x)	1m 19s
test-on-windows (ocaml.5.3.0,ocaml-option-mingw)	11m 48s
	26m 42s

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