Add failing test case and fix for incorrect chunk merges

[cmdcolin]

It can be seen in 1.0.20 and 1.0.21 that there are breaks in long nanopore features

The "completely resolved chunk" merging caused this. The test case here demonstrates it albeit with a sort of large test file of ~14mb

Potentially we need to review yet the other chunk merging case

[codecov]

Codecov Report

Merging #36 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   81.49%   81.35%   -0.14%     
==========================================
  Files          12       12              
  Lines         935      928       -7     
  Branches      196      195       -1     
==========================================
- Hits          762      755       -7     
  Misses        148      148              
  Partials       25       25

Impacted Files	Coverage Δ
src/bai.js	97.12% <ø> (-0.14%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update db4ef17...b84c90d. Read the comment docs.

[cmdcolin]

I also wanted to see how this "test" performs in the old version, with chunk merges, and with this version, with the worry that the merging was more efficient. From what I can tell this version is more efficient

Old version 1.5s, unzips ~80 chunks
New version 1s, unzips ~130 chunks

So new code is still modestly faster. It does make one wonder if there is a way to speed up the unziping, but as above, the chunk merging is not apparently a bad thing (tm)

[cmdcolin]

Probably same code will need to go into tabix-js