Replace core language HexStrings with library entity

[Walter Bright via Digitalmars-d]

On 3/15/2015 2:34 PM, Walter Bright wrote:
> P.S. Also need to include all the examples in the dlang reference as unittest
> cases.

Also, HexStrings can handle w and d postfixes. Use the lexer.c code for 
TOK::hexStringConstant() as a guide to be sure all the permutations are covered.