Fail when writing key lengths >= 256 bytes to Store

[mappum]

This is an easy safety fix to prevent a case where a key >= 256 bytes written to the store will cause the entire block to fail when writing to merk.

Will wait to merge this after #79 (will require changing the returned error) in the interest of not keeping that PR open forever.

Closes #61

In the future, we can do a better form of checking at compile-time once ed has some sort of const generic length encoding length fields.

[codecov-commenter]

Codecov Report

Merging #90 (b3a4984) into develop (b7ed050) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop      #90      +/-   ##
===========================================
- Coverage    55.89%   55.89%   -0.01%     
===========================================
  Files           39       39              
  Lines         3136     3138       +2     
===========================================
+ Hits          1753     1754       +1     
- Misses        1383     1384       +1     

Impacted Files	Coverage Δ
src/store/store.rs	94.73% <50.00%> (-1.63%)	⬇️

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