add a simple container for HashStrings

add a simple container for HashStrings

APT supports more than just one HashString and even allows to enforce
the usage of a specific hash. This class is intended to help with
storage and passing around of the HashStrings.

The cherry-pick here the un-const-ification of HashType() compared to
f4c3850ea335545e297504941dc8c7a8f1c83358. The point of this commit is
adding infrastructure for the next one. All by itself, it just adds new
symbols.

Git-Dch: Ignore