* btl/pervasive.typer: Redefine lambda as a macro

* btl/pervasive.typer (List_foldr, List_find): New functions.
(Sexp_error): Move.
(Sexp_to_list, multiarg_lambda): New functions.
(lambda_->_, lambda_=>_, lambda_≡>_): Redefine to add multiarg support
via macros.
(List_head, List_map, List_foldl):
* btl/builtins.typer (Eq_comm, Macro_expand): Don't use multi-arg
lambdas (yet).