Changed Lambda Expression
Remove Operator from Expr
Comment
Rename `Var Name` Expression constructor -> `Atom Name`
Rename `List [Expr]` constructor -> `Array [Expr]`
Refactoring
Added `Pair (Expr , Expr)` value expression type
Merge branch 'parse-expression'

* parse-expression:
  Added `Pair (Expr , Expr)` value expression type
  Refactoring
  Rename `List [Expr]` constructor -> `Array [Expr]`
  Rename `Var Name` Expression constructor -> `Atom Name`
  Comment
  Remove Operator from Expr
  Changed Lambda Expression