math gadgets refactory, for better code structure and unit test coverage.

[smtmfft]

The unit test task was initialized by #586, and we already created a unit test framework PR #865 . However, as the comment tells.

@smtmfft IMO we should also refactor the math_gadget module into modules per gadget in it, mainly to lower the lines-of-code in the entire module. What do others think about this? If yes, I'm happy to do this in a separate PR, or this can be done within #865 as well.

Originally posted by @roynalnaruto in https://github.com/privacy-scaling-explorations/zkevm-circuits/issues/586#issuecomment-1296590176

Now, the math gadgets part should be refactoryed for better code structure. So I create this task issue.

[smtmfft]

To avoid conflict, if nobody else is now working on the refactory, we will do that as we already implemented the math gadget unit test. Otherwise, we can merge the unit test into the refactory branch.

[ChihChengLiang]

Completed by #920