Add a poisonable `Barrier` to `picos_std.sync` #4709

Job	Run time
build-on-lower-bound (ubuntu-latest, 4.12.x)	1m 14s
test-on-windows (ocaml.5.2.1,ocaml-option-mingw)	13m 26s
test-on-windows (ocaml.5.3.0,ocaml-option-mingw)	12m 18s
	26m 58s

Provide feedback