debian: create missing symlinks

`rust-coreutils` provides `hashsum` as an implementation for all `*sum`
executables (md5sum, sha1sum...). As most software expect individual
binaries for each of those, this commit adds symlinks pointing to
`hashsum` for all supported algorithms.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>

debian/rust-coreutils.links

+usr/bin/hashsum usr/bin/b2sum
+usr/bin/hashsum usr/bin/md5sum
+usr/bin/hashsum usr/bin/sha1sum
+usr/bin/hashsum usr/bin/sha224sum
+usr/bin/hashsum usr/bin/sha256sum
+usr/bin/hashsum usr/bin/sha384sum
+usr/bin/hashsum usr/bin/sha512sum
+usr/bin/hashsum usr/bin/sha3sum
+usr/bin/hashsum usr/bin/sha3-224sum
+usr/bin/hashsum usr/bin/sha3-256sum
+usr/bin/hashsum usr/bin/sha3-384sum
+usr/bin/hashsum usr/bin/sha3-512sum
+usr/bin/hashsum usr/bin/shake128sum
+usr/bin/hashsum usr/bin/shake256sum