*: Using sha512 instead of sha256

sha512 is faster on 64bit systems than sha256, so prefer it over sha256

fixes #187 
@peterbourgon We have tinkered before with having `golint` in our test script to catch these things but found it way too noisy with false positives - don't suppose you have any tips for alternatives or ways to scope it more tightly? (`min_confidence` doesn't seem very helpful)