In an effort to make the `hashes` crate more ergonomic to use add a
bunch of alias' to the crate root - use re-exports where possible and
type alias' where required.
We intentionally do not rename the `foo::Hash` types so that uses have a
choice of either using the module path to differentiate or to use the
alias.
Update the crate level docs to use the alias' because they are more
terse with no loss of clarity.
264d080054