* src/lexp.ml: Various tweaks to prepare for DeBruijn indices

(db_index, db_offset, db_revindex): New types.
(vref): Use them.
(lexp): Add Shift.
(env_type): New type.
(env_lookup_type): New function.
(senv_type): New type.
(senv_lookup): New function.
(lexp_p_infer, lexp_p_check): New functions.