Layout_builders: introduce posets for better performances

Stats for nitg --separate ./nitg.nit
    before:
	0:40.04 elapsed,
	39.33 user,
	0.90 system

    after:
	0:27.58 elapsed,
	26.87 user,
	0.90 system

Average on three samples collected with GNU time. .nit_compile cache is cleaned between each run.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>