Add default constructors to Object and System.
Fixes #46
Along with 6cd753e1c2ad16b74451c8404ef3e77baf3fb05f, fixes formal
parameter type checking when redefining a method.
Fixes #43.
Fixes class type compatibility checking when assigning.
Fixes #50
Merge pull request #54 from lunaskyrise/pixie-dust-tree

Merges some more fixes for PixieDustTree