Characters should be unboxed

[jcubic]

This was a mistake in #233

(let ((x (vector)))
  (x.push #\x)
  x)

It's ok for Array::push to unbox the values, and characters can be unboxed to strings.

If you want to have character vectors, you can use set-obj! on array.

(let ((x (vector)))
  (set-obj! x x.length #\x)
  x)

It should also be documented to be careful with native methods because they always unbox the values.