Added Integer as an instance of Display type-class
Added tyarith (Chapter8) foundation data-types
Enhance NumVal data-type as an instance of (Ord, Enum, Monoid)
Split out NumVal data-type from Syntax.hs
Added HasType type-class
To be NumVal as an instance of HasType (any-value of NumVal has TyNat type)
Added Display test case (Integer)
Added Tests about TypeError data-type
To be Term as an instance of HasType (Term include Mismatch TypeError)
Added Tests about Term data-type
#1 Implement Term data-type as an instance of Display type-class
Merge pull request #2 from VoQn/Chapter8

(Chapter 8) Added tyarith basic syntax types