vmstack: ensure consistent ordering of internal slice

[hsslbchtop]

VmStack is represented as Golang slice []VmStackValue. But notion of where is the top of the stack was different on Marshalling (top of the stack is stack[0]) and Unrmashalling (top of the stack goes to stack[len]).

This PR addresses issue by changing VmStack type from slice to struct, with the []VmStackValue slice incapsulated into private property. This interface will ensure that stack is only manipulated using interface functions (Len, Peek, Put), and the underlying slice is not accessible for direct manipulation.

Migration to interface is following:

• len(stack) becomes stack.Len()
• index stack[0] was ambiguos and could have both meanining - top and bottom of the stack:
  • if stack was Unmarshalled, then stack[0] was meant to access bottom of the stack, hence Peek(len-1) should be used instead
  • if stack was constructed (usually in order to be marshalled later), then stack[x] == Peek(x)
  • similarly if stack was created using slice stack := []VmStackValue{a,b,c} (in this case a meant to be the top of the stack), then Put operations in reverse order should be done: stack.Put(c); stack.Put(b); stack.Put(a)