Add CPU trellis decoder

[noajshu]

Summary

Adds a tesseract_trellis decoder binary and library.
I built this on a branch and opened a PR earlier (#230) but closed it because it grew too big and ocmplicted.

But I do need this tesseract trellis binary to live somewhere and anywhere now we should land it.
In the meantime at QEC26 I saw Leverrier's talk on the frontier decoder which is quite similar conceptually. There are some differences but anyways it is a nice idea and I look forward to comparing to the different future cost estimates in their paper.

Usage example:

bazel build src:tesseract_trellis

./bazel-bin/src/tesseract_trellis \
  --circuit path/to/circuit.stim \
  --sample-num-shots 1000 \
  --sample-seed 1234 \
  --threads 1 \
  --ranking-mode future-detcost \
  --beam 1024 \
  --obs-probs-out obs_probs.bin

I highly recommend to use future-detcost ranking mode to allow good performance at the smallest beam sizes.

[mhucka]

This is not as throughout a review as it should be; this is a long PR and I'm kind of out of my depth on the technical and C++ details. Still, I did what I could. Hopefully the comments are more helpful than annoying …

[noajshu]

This is not as throughout a review as it should be; this is a long PR and I'm kind of out of my depth on the technical and C++ details. Still, I did what I could. Hopefully the comments are more helpful than annoying …

Thank you @mhucka ! This is very helpful. I addressed many of the comments.

[mhucka]

One last detail. Optional. Otherwise LGTM!

[noajshu]

Thanks again for the review @mhucka !