How to refer to different sized static arrays

[Adnan]

Just a foreword, this is for learning purposes, hence I am not 
using the dynamic array or Array!T.

I have a structure that maintains a heap allocated sized array 
inside.

struct LifoStack(T) {
   T[?] data;
}

This `data` is manually resized and copied. Thus the size itself 
is not a compile time constant. What should go inside `?` in the 
the type signature of data?

Also, how can I create runtime-determined sized fixed array in 
the heap?