V1 migration

[mappum]

This PR makes so Merk stores track their format version in the internal column family, and migrates from the V0 format (u8 key lengths) to V1 (u16 key lengths) when opening.

[codecov]

Codecov Report

Attention: Patch coverage is 18.89764% with 103 lines in your changes missing coverage. Please review.

Project coverage is 92.48%. Comparing base (058839b) to head (aab43ff).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/merk/mod.rs	25.53%	70 Missing ⚠️
src/tree/encoding.rs	0.00%	19 Missing ⚠️
src/tree/link.rs	0.00%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #82      +/-   ##
===========================================
- Coverage    94.21%   92.48%   -1.73%     
===========================================
  Files           26       26              
  Lines         5464     5592     +128     
===========================================
+ Hits          5148     5172      +24     
- Misses         316      420     +104     

Files with missing lines	Coverage Δ
src/tree/link.rs	90.15% <0.00%> (-4.20%)	⬇️
src/tree/encoding.rs	87.65% <0.00%> (-11.65%)	⬇️
src/merk/mod.rs	80.84% <25.53%> (-9.30%)	⬇️