Fix inf loop on too long input, plus a small simplification with more comments

[MrZ626]

Now it throws an error saying "too long input data"

[MrZ626]

sorry for finding a bug (?) just after I thought everything finished lol

[pgundlach]

Do you have an example for a "too long" string? Perhaps it is worth putting in the blackbox data?

[MrZ626]

("z"):rep(2953) works, ("z"):rep(2954) fails

• in old version, 2953 have output, 2954 inf loop
• after my fix, 2953 have output, 2954 error

edge case passed, so I think this is a correct implementaion

[pgundlach]

Possible commit message, please change if you see anything wrong with it:

Fail fast on oversize QR data and simplify block weaving

- assert when no QR version fits instead of looping forever
- inline xor lookup usage and adjust tests
- streamline data/EC interleaving; add max-length blackbox case