The crypto package is added as a common place to store identifiers for
hash functions. At the moment, the rsa package has an enumeration of
hash functions and knowledge of their digest lengths. This is an
unfortunate coupling and other high level crypto packages tend to need
to duplicate this enumeration and knowledge (i.e. openpgp).

crypto pulls this code out into a common location.

It would also make sense to add similar support for ciphers to crypto,
but the problem there isn't as acute that isn't done in this change.

R=bradfitzgo, r, rsc
CC=golang-dev
https://golang.org/cl/4080046