* src/opslexp.ml (lookup_type, lookup_value): Move to debruijn.ml

* src/debruijn.ml (_get_env): Remove functions.
(env_lookup_by_index, print_lexp_ctx): Change ctx type.
(lctx_lookup): Rename from _env_lookup; change ctx type.
(lctx_lookup_type, lctx_lookup_value): Move from opslexp.ml; use
lctx_lookup.
(env_lookup_type, env_lookup_expr): Use them.