Replace ShortHash with MD5 in order to avoid hash collisions in Mempool tests

[nfrisby]

We have observed a ShortHash collision in the ouroboros-consensus:test:test-consensus:Test.Consensus.Mempool module. It uses ShortHash. That implementation appears to have the same compute time as MD5, but a quarter of the bits (it just takes a prefix of the MD5).

Instead of writing the tests to anticipate the occasional collision and thereby save the debugging time whenever it happens, we're just going to switch to MD5, under the assumption that it won't collide in these tests during the project's lifetime.

Thomas summarized the change as follows.

sed -i s/ShortHash/MD5/g ouroboros-consensus/ouroboros-consensus-mock/src/Ouroboros/Consensus/Mock/Ledger/UTxO.hs