Use ##... for types. Don't use `Macro` for special forms

* btl/unit.typer, btl/list.typer, btl/macro.typer: Remove, unused.
* btl/builtins.typer: Rename from types.typer.
(Unit): Define as inductive, rather than builtin.
(unit): New predefined constant.
(Int, Float, String, Sexp, Context, IO, FileHandle, Attribute):
Remove built-in types, now declared in OCaml.
(new-attribute, get-attribute, has-attribute, add-attribute, decltype)
(declexpr): Remove special forms, now declared in OCaml.

* src/builtin.ml (predef_name): Remove "Attribute", unused.
(add_builtin_type): New function.
Add all builtin sorts and types.

* src/debruijn.ml (sort_level): Rename from type_level.
(sort_omega): Rename from type_omega.
(type_level): New type.

* src/lparse.ml (macromap): Move.
(special_forms): Rename from make_macro_map; make it a ref cell rather
than a function of no arguments.
(type_special_form): New built-in type.
(add_special_form): New function.
(get_special_form): Rename from get_macro_impl, rewrite.
(infer_call.handle_funcall): Use mkBuiltin.  Remove the single-arg form
of `Built-in`.
(infer_call): Don't use the `Macro` to indicate a special form.
(builtin_macro, is_builtin_macro): Remove.
(<toplevel>): Register special forms.
(default_lctx): Add entries predefined in BI.lmap. before loading
types.typer.

* src/opslexp.ml (conv_p) <Inductive>: Fix scoping of fields.
(sort_level_max): Hash-cons return value.
(check'): Check the type provided to `Builtin`.
Hash cons the sorts as well.
(erase_type): Silence compiler warning.