des: tweak weak key test

[newpavlov]

The new code results in a slightly better codegen while still being const time, including 32 bit targets: https://rust.godbolt.org/z/K7cjj5d1P

[newpavlov]

@baloo
I am not sure what exact purpose of the bit counting loop was, so I removed it for now. Could you explain it?

[baloo]

That was a copy paste from https://github.com/iqlusioninc/yubikey.rs/blob/13bdf9a5850c124057825297b51eaecaf8ebe9e2/src/mgm/tdes.rs#L36
I think @str4d was the original author.

[newpavlov]

I added subkey equality check for TDES in #470.

[baloo]

I believe this changed the behavior of the test (iqlusioninc/yubikey.rs#639)

(or #470 not sure)

[newpavlov]

@baloo
Could you open a new issue for it with reproduction code? The current code simply checks every subkey against the list of weak DES keys. It may not be the recommended approach for TDES, but then we would need a proper specification of the check.