Add checksum helper functions.

 Add checksum helper functions.
 
 These functions make it easier to write code that wants to compute a
 checksum for some data while allowing the user to configure the type
 of checksum that gets used.
 
 This is another piece of infrastructure for the upcoming patch to add
 backup manifests.
 
 Patch written from scratch by me, but it is similar to previous work
 by Rushabh Lathia and Suraj Kharage. Suraj also reviewed this version
 off-list. Advice on how not to break Windows from Davinder Singh.
 
 Discussion: http://postgr.es/m/CA+TgmoZV8dw1H2bzZ9xkKwdrk8+XYa+DC9H=F7heO2zna5T6qg@mail.gmail.com
 Discussion: http://postgr.es/m/CA+TgmoZRTBiPyvQEwV79PU1ePTtSEo2UeVncrkJMbn1sU1gnRA@mail.gmail.com