Simplify arg sep logic.
Work on expressions with functions.
Move precendence logic into Node.
Hard-code 'add' function with 2 args working.
Add notion of builtin functions.
Remove InvalidTokenException.
Make token and type properties on Node readonly.
Move 'rightAssociative' method into ASTBuilder since it is only required when building the AST.
Simplify prefix traversal.
Move preorder node collection logic into ASTBuilderTest, since that is the only client using that method.
Replace node.left and node.right with node.childNodes.  This is in preparation for supporting vararg functions.
Rename node.childNodes -> node.children.