Cleanup XorPlus8 + guard against infinite loop in table construction

[steve-scalyr]

This PR contains some minor changes to the XorPlus8 class:

• Update / clarify a few comments.
• More consistent use of this. when assigning instance fields in constructor
• Round the table length down to a multiple of 3, as any additional entries are wasted
• More consistent computation of bitCount; remove an unused constructor that was assigning it incorrectly
• Guard against infinite loops during table construction (could be caused by code bugs or bad input)

Regarding the unused constructor that I removed: it did not populate seed or rank, and I believe the latter means that any attempt to use the instance created by this constructor would have resulted in NullPointerException.

The changes are small, but can most easily be reviewed one commit at a time.

[steve-scalyr]

@thomasmueller could you take a look? This PR contains the infinite loop guard proposed in the discussion of https://github.com/FastFilter/fastfilter_java/pull/30. I also took the opportunity to read through the code to make sure I understood all of it, and along the way I made a few other small changes.

[steve-scalyr]

Thanks!

One last request: would it be possible to publish a new release (1.0.3) now?