Add a poisonable `Barrier` to `picos_std.sync` #4703

Job	Run time
build-on-lower-bound (ubuntu-latest, 4.12.x)	1m 22s
test-on-windows (ocaml.5.2.1,ocaml-option-mingw)	13m 4s
test-on-windows (ocaml.5.3.0,ocaml-option-mingw)	12m 3s
	26m 29s

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