Layout_builders: introduce posets for better performances

    Stats for nitg ./nitg.nit
    before:
	0:21.23 elapsed,
	20.68 user,
	0.64 system

    after:
	0:18.50 elapsed,
	17.97 user,
	0.62 system

Average on 5 samples collected with GNU time.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>