Streamline the integer binary ops slightly

* btl/builtins.typer (_+_, _*_): Adapt to new builtin names.
(_-_, _/_): New builtins.

* src/eval.ml (add_binary_iop): Rename from _generic_binary_iop;
Rewrite.  Register the primitive into builtin_functions.
(iadd_impl, isub_impl, imult_impl, idiv_impl): Don't give them names.
(typer_builtins_impl): Remove.
(<toplevel>): Don't manually add _+_ and _*_ to builtin_functions.

* src/lparse.ml (sform_built_in): Check builtin_functions to make sure
the built-in actually exists.