Speed up the per-block loops of AES, DES3, and DES IOV encryption by

Speed up the per-block loops of AES, DES3, and DES IOV encryption by
avoiding function calls and copies in the case where the next block
is wholly contained within the current buffer.  To do this, introduce
two new inline functions in aead.h called iov_next_block and
iov_store_block.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23430 dc483132-0cff-0310-8789-dd5450dbe970