Add "fin" constraint on index size for (@), (!) and (!!), like (@@) has.

Prior to this changeset, any of the following three expressions

[1,2,3] @ (zero : [inf])
[1,2,3] ! (zero : [inf])
[1,2,3] !! [zero : [inf]]

would produce the following error:

  Location:  [Eval] fromVWord
  Message:   not a word
             [False, False, False, False, False, ...]