ng: implement iota

[sbinet]

the following fails in ng:

ng> type Options int32
ng> const (
..> OptNone      Options = 1 << iota // default
..> WithXErrBars                     // enable display of X-error bars (if available)
..> WithYErrBars                     // enable display of Y-error bars (if available)
..> )
ng: typecheck: undeclared identifier: iota