Constify the AES code and propagate to consumers.  This allows us to
update the Fortuna code to use SHAd-256 as defined in FS&K.

Approved by:	so (self)
I just realized that the previous commit message makes no sense: the
first sentence should have read "Constify the AES and SHA-256 code and
wrappers".  This allows us to feed zero_region (which is const) to the
hash function during reseeding and thereby implement the FS&K version of
SHAd-256 instead of the older F&S version.

Approved by:	so (self)