Church Numerals filled in

Filled in Church Numerals content.

Also, I changed some of the code blocks from:
\`\`\`\`
data Exp = ...
       | Let String Exp Exp
\`\`\`\`

to:
\begin{code}
data Exp = ...
     | Let String Exp Exp
\end{code}

Reason: On Linux, code inside four backticks turned into \texttt{}, and the pipe characters were interpreted by lhs2TeX as beginning inline code. So lhs2TeX would interpret
\texttt{data Exp = ...                  | Let String Exp Exp}
as starting literate Haskell with `| Let String Exp Exp} ...`. lhs2TeX would complain that there was not a matching ending pipe for the supposed inline code.