Modification to safeEval.
Added solveTask method which generates a program plan greedily.
Remove solveTask2 (debuggin method).
Fixed safeEval and moved cBottom (the standard bottom element) from the Library.hs to Expr.hs.
Removed cBottom from Library.hs. Added compose function which takes a list of expressions and returns a single expression of their compositions.
Simplified the methods which sample expressions from a library.
Added a method which generates a task which returns the closeness of the input expression to the target integer.