AST/ASR printing improved

Fix #304 (closed)

Resultant Output:

$ lfortran --show-asr --indent examples/expr2.f90 
(TranslationUnit 
   (SymbolTable 
      1 
      {
         expr2: 
            (Program 
               (SymbolTable 
                  2 
                  {
                     x: 
                        (Variable 
                           2 
                           x 
                           Local () 
                           Default 
                           (Integer 4 []) 
                           Source 
                           Public)
                  }) 
               expr2 [
               (= 
                  (Var 2 x) 
                  (BinOp 
                     (BinOp (ConstantInteger 2 
                        (Integer 4 [])) 
                        Add (ConstantInteger 3 
                        (Integer 4 [])) 
                        (Integer 4 [])) 
                     Mul (ConstantInteger 5 
                     (Integer 4 [])) 
                     (Integer 4 []))) 
               (Print () [
                  (Var 2 x)])])
      }) 
   [])