Introduce ratio operation

Just like in Physics, the ratio of 2 things in the same unit, should be
without unit.

Example use case:
- ratio of a Depth by a Depth (eg. ONE_PLY) should be an int.
- ratio of a Value by a Value (eg. PawnValueEg) should be an int.

Remove a bunch of useless const while there.

No functional change.
Besides, the optimization you propose can be done by the compiler. The reason the compiler is allowed to do it, is that signed int overflow is undefined.