Create missing directories in the LMDBDict constructor.

[maleadt]

LMDB's mdb_env_open requires the environment directory to already exist, so following the README example with a fresh path failed with LMDBError(2): No such file or directory. Since LMDBDict presents a persistent Dict abstraction, create the directory on writable opens; readonly opens still error on a missing path.

Fixes #76

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (0a0ea8d) to head (6372c76).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   80.62%   80.64%   +0.02%     
==========================================
  Files           9        9              
  Lines         836      837       +1     
==========================================
+ Hits          674      675       +1     
  Misses        162      162              

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