MAJOR refactor.  Moved all declarative classes into their own subpackage
(so there are now three categories of domain classes: declaration,
statement, and expression).  Also separated Variable into
VariableDeclaration and VariableExpression.  Also added ProgramContext
and a LinkedList of them in GeppettoProgram, and moved the symbol table
into it.  This will allow us to do variable scoping.  Neat!