Declare `[]` iterator

[haxscramper]

In my real use case I have to collect subnodes into sequence using calls to C 
procedure, so I _have_ to either build full sequence, or provide an iterator. 
Sequence solution also performs worse for code like
    
    
    for sub in node[0 .. ^1]:
      if sub.matches(< ... some predicate ... >):
        # do things
        break
    
    
    Run

I have to pay for the whole sequence, while I might actually need only first 
couple of elements.