Split code into modules

Also did some work on laws

Calculator.hs

@@ -0,0 +1,22 @@
+module Calculator where
+
+import Utils
+import Expr
+import Parser
+
+data Law = Law LawName Equation
+ deriving Show
+
+type LawName = String
+type Equation = (Expr,Expr)
+
+law :: Parser Law
+law = do name <- upto ':'
+         eqn <- equation
+         return (Law name eqn)
+
+equation :: Parser Equation
+equation = do lh <- expr
+              symbol "="
+              rh <- expr
+              return (lh, rh)