Move EvalDoubleVisitor to eval_double.h/cpp
Put EvalDoubleVisitor into .cpp
Simplify the EvalDoubleVisitor implementation
Remove the old comment
Avoid instantiation of EvalDoubleVisitor

Add a comment explaining how it works. Now only one instance of
EvalDoubleVisitor needs to be constructed for the whole symbolic evaluation.
Add eval_double.h into headers
Merge pull request #300 from certik/eval

Put EvalDoubleVisitor into its own file