Replace core language HexStrings with library entity

[Walter Bright via Digitalmars-d]

HexStrings:

    http://dlang.org/lex.html#HexString
They're rarely used, but very useful when needed. But, as the octal literals 
have shown, they can be easily replaced with a library template:

    x"00 FBCD 32FD 0A"

becomes:

    hex!"00 FBCD 32FD 0A"

It'll simplify the core language slightly.

Thoughts? Anyone want to write the hex template? Any other ideas on things that 
can removed from the core language and replaced with library entities?