Introduce DatabasePaths for valid filename inputs

[robo9k]

This implements #112 (comment)

It does not implement a builder as in #6 but using the default database should now be more obvious.
This should thus help with #22 as well.

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (2fc648b) 39.04% compared to head (4155f6a) 40.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   39.04%   40.88%   +1.84%     
==========================================
  Files           2        2              
  Lines         584      587       +3     
  Branches      584      587       +3     
==========================================
+ Hits          228      240      +12     
+ Misses        353      346       -7     
+ Partials        3        1       -2     

Files	Coverage Δ
src/lib.rs	38.15% <55.00%> (+2.72%)	⬆️

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