Basic support for insert with on conflict

[Fs02]

Implement go-rel/rel#217

[codecov]

Codecov Report

Merging #26 (95f05db) into main (524d6d3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          128       129    +1     
=========================================
+ Hits           128       129    +1     

Impacted Files	Coverage Δ
postgres.go	100.00% <100.00%> (ø)

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