Remove Connection path field

[gwenn]

Use sqlite3_db_filename instead.
Pros: memory footprint, absolute path
Cons: one cannot make the disctinction between multiple memory databases anymore (like file:one.db?mode=memory and file:another.db?mode=memory).

[codecov]

Codecov Report

Base: 66.00% // Head: 66.04% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (8a28167) compared to base (cc2c290).
Patch coverage: 87.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1248      +/-   ##
==========================================
+ Coverage   66.00%   66.04%   +0.04%     
==========================================
  Files          45       45              
  Lines        3759     3752       -7     
==========================================
- Hits         2481     2478       -3     
+ Misses       1278     1274       -4     

Impacted Files	Coverage Δ
src/lib.rs	65.23% <87.50%> (+0.65%)	⬆️

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