htools: adjust imports for newer compilers

While testing with ghc 7.2, I saw that some imports we are using are
very old (from ghc 6.8 time), even though current libraries are using
different names.

We fix this and bump minimum documented version to ghc 6.12, as I
don't have 6.10 to test anymore (possibly still works with that
version, but better safe - both Ubuntu Lucid and Debian Squeeze ship
with 6.12 nowadays).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>