Teach rustc about iterators

[graydon]

Rustc has no support for iterators beyond basic parsing. Neither the iterator nor the for each loop that interlocks with it. Implement these.

[graydon]

Works if they don't do environment access as of cset b0c7439. Will followup for the env access case.

[pcwalton]

Everything except iter-ret now works; filing a followup for this.

[pcwalton]

Filed issue #270. Closing this as iterators now work otherwise.