exp/ssa: special-case 'range' loops based on type of range expression.

The lowering of ast.RangeStmt now has three distinct cases:

1) rangeIter for maps and strings; approximately:
    it = range x
    for {
      k, v, ok = next it
      if !ok { break }
      ...
    }
   The Range instruction and the interpreter's "iter"
   datatype are now restricted to these types.

2) rangeChan for channels; approximately:
    for {
      k, ok = <-x
      if !ok { break }
      ...
    }

3) rangeIndexed for slices, arrays, and *array; approximately:
    for k, l = 0, len(x); k < l; k++ {
      v = x[k]
      ...
    }

In all cases we now evaluate the side effects of the range expression
exactly once, per comments on http://code.google.com/p/go/issues/detail?id=4644.

However the exact spec wording is still being discussed in
https://codereview.appspot.com/7307083/.  Further (small)
changes may be required once the dust settles.

R=iant
CC=golang-dev
https://codereview.appspot.com/7303074