Using bitfields to add 12 more previously wasted  bits to the key stored...

... in TTentry.

This greatly reduces the likelihood of hard hash collisions and all the very difficult
to tract bugs potentially resulting from them.  I have verified in the debugger and some
extra code that this does actually prevent some collisions that happen in practice.
The standart bench doesn't search enough nodes to run into a collision so the signature remains the same.
However if you run:
stockfish bench 32 1 24 default depth  (this takes over 10 minutes on my machine)

you will see the signature change compared to the current master due to collision.
No functional change otherwise.