Add syntactic sugar "class"

[sychoo]

The "class" declaration will combine object type declaration and object implementation.

i.e. type IntList def sum():Int val empty:IntList = new def sum():Int = 0

is equivalent to class empty:IntList = new def sum():Int = 0

[JonathanAldrich]

One potential design for this is outlined in this paper: http://www.cs.cmu.edu/~aldrich/papers/maspeghi13.pdf