* src/lparse.ml (lexp_call): Rewrite to work one-arg at a time

Add basic support for explicit-implicit args.  Use lexp_whnf.
(lexp_call.get_return_type): Remove.
(_lexp_p_infer, lexp_read_pattern, lexp_decls_macro):
Adjust to new env_lookup_expr.
(_lexp_parse_all.loop): Don't use List.hd when not needed.
* src/typecheck.ml (lexp_whnf): New function.
* src/debruijn.ml (env_lookup_expr): Return an option.
* src/builtin.ml (get_attribute_impl, declexpr_impl): Adjust accordingly.
* btl/types.typer (length): One more explicit-implicit arg.