Removed divison operations

Removed divison operations

This has no effect on most platform with most modern compiler, and makes
the code slightly less readable to boot.

But.

Some compilers may fail to transform divisions by a power of two into
the relevant shift or mask.  Moreover, some platforms sport a variable
time division operation.

In the name of safety against timing attacks, those operation have been
removed explicitly.  Only one remains, in Argon2i, but its operands are
not secret.