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