Replace Bindlib by de Bruijn indices

[fblanqui]

Comparison on 22/02/24 of db branch 60945055 with master branch 9597727 on Intel(R) Core(TM) i7-8665U CPU @ 1.90GHz with 32Go RAM and ocaml 4.14.1:

library	master	db	%
holide	5m29s	6m58s	+27%
matita	3m57s	Out of memory

Remark: out of memory with commits 6babc67, 6babc67, 7862dd84, ff0f98e

TODO:

• [ ] fix memory blow up
• [ ] fix handling of tests/OK/unif_hint.lp

[fblanqui]

Effect on efficiency:

library	master	PR	%
focalide	18s	29s	+61%
holide	5m57s	7m6s	+19%
matita	4m35s	4m12s	-8%