Make the DenseMap bucket type configurable and use a smaller bucket for DenseSet.

DenseSet used to be implemented as DenseMap<Key, char>, which usually doubled
the memory footprint of the map. Now we use a compressed set so the second
element uses no memory at all. This required some surgery on DenseMap as
all accesses to the bucket now have to go through methods; this should
have no impact on the behavior of DenseMap though. The new default bucket
type for DenseMap is a slightly extended std::pair as we expose it through
DenseMap's iterator and don't want to break any existing users.
Turn some DenseMaps that are only used for set operations into DenseSets.

DenseSet has better memory efficiency now.