Convert 'Hashdist' to the canonical 'HashDist' capitalization

With:

  $ sed -i 's/Hashdist/HashDist/g' $(git grep -l Hashdist)
  $ sed -i 's/HashDistdoc/HashDistDoc/g' $(git grep -l HashDistdoc)

This might create silly conflicts with outstanding branches.  They
should be trivial to resolve, but it's also easy for me to re-run the
seds after outstanding PRs land.