Making `Turtle.xcor` and `Turtle.ycor` accessible without the getter

Here's my policy on this stuff:

  * If a varname begins with '_', it's Private.  Private things cannot
    be read by outsiders.  There are a few exceptions to this (e.g.
    `Topology` has several Private abstract methods that are really
    Protected; `Updater` reads Private data from `Updatable`s).
    Generally, though, Private data cannot be read or written by outside
    sources.  Private things are internal details of sorts.

  * If a varname doesn't begin with '_', it's Public.  Public things
    _can_ be read by outsiders, but they cannot be written to.  If you
    want to write data into an object, call one of its functions (e.g.
    `Turtle.setVariable`).  Simply overriding the value of an outside
    variable or method is not supported and really ought to never be.

If these rules are followed, then variables don't need to be hidden
behind senseless getters.  If you don't want to follow the rules, this
is JavaScript, so it's not like I could stop you, even if I really
wanted to.