Itcl_InitStack initializes a stack structure and Itcl_DeleteStack
deletes it. Itcl_PushStack pushes the cdata value onto the stack.
Itcl_PopStack removes and returns the top most cdata value.
Itcl_PeekStack returns the top most value, but does not remove it.
Itcl_GetStackValue gets a value at some index within the stack. Index
"0" is the first value pushed onto the stack. Itcl_GetStackSize
returns the count of entries on the stack.