Fixed-precision Ints

Now supporting Int8, Int16, Int32 and Int64.
The spec says that an Int has an unspecified precision of at least
2^29-1, but on my machine they are Int64, so I placed Int above Int64 in
my numeric tower. If your machine behaves differently, all you have to
do is to reorder the list, and all the correct promotions will be
re-generated.