Make Column iterable. - #81

This exposes a [Symbol.iterator] method on Column simply by passing through to the underlying data. This means that you can iterate over column values slightly more succinctly (no .data required), for example:

for (const value of table.column("foo")) {
  console.log(value);
}

Or to make a copy of a column:

const foo = Array.from(table.column("foo"));