Removed old haddock reader code.  Add dependency on haddock-library.

This also removes the dependency on alex and happy.
Travis:  don't need alex, happy.
Rewrote haddock reader to use haddock-library.

This brings pandoc's rendering of haddock markup in line
with the new haddock.

Note that we preserve line breaks in `@` code blocks, unlike
the earlier version.

Modified tests pass.  More tests would be good.
Finished first draft of Haddock writer.
Added haddock template.
More polish on Haddock reader/writer.
Added haddock writer tests.
Revised haddock reader tests for changes in reader.
Small fix to haddock "tables".
Haddock writer:  Only use Decimal list style.
Updated haddock writer tests.