Allow to iterate over hashes, closes #139

[odino]

for k, v in {"a": 1, "b": 2, "c": 3} {
  echo(k, v)
}

This required a bit of refactoring on the iterables,
but nothing too crazy. I like how in this new implementation we check whether we're done iterating when the key is nil, rather than when it's < 0 which is wonky.

[ntwrick]

Very useful expression.

In file for.md, the example on line 66 would be more consistent with line 79 if it were for v in {...} instead of for x in {...} since you are iterating over the values.