Added the concept of 'attributes'.

Attributes are analogous to data fields in Java. For example:

    x := + 3i 1

    println => x 're'
    println => x 'im'

creates a variable `x` consisting of `3i + 1`. Next, the real part and
the imaginary part of `x` are printed.

Parts of the language will be reworked shortly to take full advantage
of this feature.