macro for allocating closures to the heap. Heap-allocated closures can be convenient when (re)assigning closures to structure fields, though heap allocation comes at the expense of inline optimization.
Usage: ```rust let closure = heap_fn!( || { println!("This closure lives in the heap now!") } );
closure.c()(); // "This closure lives in the heap now!"
let closure_identifier = || {println!("Named closure!")};
heapfn!(closureidentifier).c()(); // "Named closure!"
```