Implement strings without value16

This fixes #68

Some changes over the original patch, removing references to utf8string.String:

* (better) This removes a dependency on a non-standard (though solid) package
* (better) utf8string.String has mutable parts
* (worse) utf8string.String has a smarter consecutive access approach (by
  remembering where the last access was)
* (?) _stringWide allocates a []rune if charAt or charCodeAt access is needed (though
  it will only do this once for the string object)