mie: ir: replace mie_module ir object with a builtin generic "module" op

the builtin.module op behaves exactly like any other op, and is designed
to be a container for other ops of any type. it can be used as the
top-level container for other ops (other top-level containers can be used
instead).

this also changes the file format. now, an ir file can only contain a
single top-level op. any other ops in the ir must be contained
directly or indirectly by this top-level op.

mie/print/module.c

@@ -1,12 +0,0 @@
-#include <mie/ir/module.h>
-#include <mie/ir/op.h>
-#include <mie/print/printer.h>
-
-void mie_printer_print_module(
-	struct mie_printer *printer, const struct mie_module *mod)
-{
-	for (size_t i = 0; i < MIE_VECTOR_COUNT(mod->m_ops); i++) {
-		mie_printer_print_op(printer, &mod->m_ops.items[i]);
-		b_stream_write_char(printer->p_stream, '\n');
-	}
-}