all: promote value type function arguments to heap if necessary

[UweKrueger]

This is a further follow-up to #9873 (and #10033). It makes the following possible:

• Value arguments of functions is moved to heap if referenced:

fn get_ref(s St) &St {
    return &s
}

• The above point always applies to structs declared as [heap] that are passed as value to a function
• automatic heap promotion now also works for other data types (not only structs) – so the following works:

fn get_heap_ref() &int {
    n := 13
    return &n
}

fn promote_to_heap(x f64) &f64 {
    return &x
}