Experimental v5 SBT format

[luizirber]

This brings the minimum v5 support from #456 (internal changes and loading from file), while still defaulting to saving to v4. This allows experimenting with the format without commiting to full support (yet).

Differences from v4 to v5:

• Internal representation uses two dictionaries: one for internal nodes, another for leaves.
• Only leaves are required to be present in the SBT file, and it will rebuild internal nodes on load.
• Prototype for insertion without updating internal nodes (delay internal node generation to save)

(v5 is the default in the rust codebase)

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #694 into master will decrease coverage by 0.09%.
The diff coverage is 89.94%.

@@            Coverage Diff            @@
##           master     #694     +/-   ##
=========================================
- Coverage   88.71%   88.61%   -0.1%     
=========================================
  Files          30       30             
  Lines        4617     4735    +118     
  Branches       45       45             
=========================================
+ Hits         4096     4196    +100     
- Misses        519      537     +18     
  Partials        2        2

Impacted Files	Coverage Δ
sourmash/sbt.py	85.53% <89.94%> (-0.19%)	⬇️

---

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 878540f...98aa5b5. Read the comment docs.

[luizirber]

Ready for review and merge @ctb

[ctb]

Go ahead and merge as you wish @luizirber !

[ctb]

Go ahead and merge as you wish @luizirber !