Fix lexp's Induction to name fields. Fix lexp_context for real vars.

* src/debruijn.ml (db_idx, db_ridx): Re-introduce those type aliases.
(senv_add_var): Remove redundancy.  Change calling convention.
(lexp_context): Make var's "value" optional.
(env_extend): New function.

* src/eval.ml (nfirst_rte_var._eval.ctor_name): Fix confusion in
constructor args.

* src/lexp.ml (lexp): Add optional name to constructor fields.

* src/lparse.ml (default_lctx): Adjust to new senv_add_var.
(lexp_parse): Consolidate the two Plambda cases.  Don't mess with
local_scope here.
(lexp_parse_inductive): Rename from lexp_parse_constructors.
Adapt to lexp changes.  Use List.fold_left.
(lexp_decls): Adjust to new senv_add_var.
(lexp_context_print): Handle vars without value.