Hex literals don't work properly

[kosmikus]

Reported by @phadej.

Failing example:

\documentclass{article}
%include polycode.fmt

\begin{document}
\begin{code}
module Hex where
bug =  let  x = 0xdeadbeef
            y = 0x12345678
       in x + y
\end{code}
\end{document}