Before this change, for each constructor that we want
to allocate a tag for we would traverse a list of all
the constructors in a datatype to determine which tag
a constructor should get.
This is obviously quadratic and for datatypes with 10k
constructors it actually makes a big difference.
This change implements the plan outlined by @simonpj in
which is basically about using a map and constructing it outside the
One place where things got a bit awkward was TysWiredIn.hs,
it would have been possible to just assign the tags by hand, but
that seemed error-prone to me, so I decided to go through a map
there as well.