WIP Refactor EntryModel (#576)

* refactor Entry model for integeter and STL API likeness.

* Add more test coverage.

* Only place leading zeros if there is room.

* Overflow protection for leading zero count.

* Access for leading zeros present.

* increment/decrement operators plus improved clamping functionality.

* Add more tests.

* remove dead code.

* Add EntryModelBounded.

* Change all comments to C++ style for consistency.

* remove lower case hex option.

* Cleanup size_ handling.

* Change logic into assert which should always be true.

* Simplify code.

* Fix other review comments.

* Fix/add tests.

* Add access method for maximum size (in digits).