Add MIR_get_global_item

[iacore]

MIR_get_global_item was in mir.h but missing definition in mir.c. This patch tries to add its definition.

[vnmakarov]

I am not against to have MIR_get_global_item and MIR_get_module_item in API.

I don't like this MIR_get_global_item. The function actually finds the first item with the name. The item may be visible only in one module (has no export clause).

The search is not efficient. I remember there is somewhere in loading/linking code more efficient way to find an external item. It would be a good starting point to implement MIR_get_global_item.

Also these two functions should be documented in MIR.md

[iacore]

I have updated the issue description.

What's the correct way to look for a function definition in textual MIR loaded with MIR_scan_string?

I took the search code from

mir/c2mir/c2mir-driver.c

Lines 786 to 793 in 643bb0c

	for (module = DLIST_HEAD (MIR_module_t, *MIR_get_module_list (main_ctx)); module != NULL;
	module = DLIST_NEXT (MIR_module_t, module)) {
	for (func = DLIST_HEAD (MIR_item_t, module->items); func != NULL;
	func = DLIST_NEXT (MIR_item_t, func))
	if (func->item_type == MIR_func_item && strcmp (func->u.func->name, "main") == 0)
	main_func = func;
	MIR_load_module (main_ctx, module);
	}