stop using MissingH for MD5

Cryptonite is faster and allocates less, and I want to get rid of
MissingH use.

Note that the new dependency on memory is free; it's a dependency of
cryptonite.

This commit was supported by the NSF-funded DataLad project.

Utility/Tuple.hs

@@ -0,0 +1,15 @@
+{- tuple utility functions
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+fst3 :: (a,b,c) -> a
+fst3 (a,b,c) = a
+
+snd3 :: (a,b,c) -> b
+snd3 (a,b,c) = b
+
+thd3 :: (a,b,c) -> c
+thd3 (a,b,c) = c