Cleanup of PostgreSQLDataType (using PgDataType instead).

[isoos]

type alias is kept in top-level postgres.dart library.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e2bdea5) 86.46% compared to head (a5b3595) 86.60%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   86.46%   86.60%   +0.14%     
==========================================
  Files          30       30              
  Lines        2696     2696              
==========================================
+ Hits         2331     2335       +4     
+ Misses        365      361       -4     

Files	Coverage Δ
lib/src/query.dart	95.62% <100.00%> (ø)
lib/src/substituter.dart	96.92% <100.00%> (ø)
lib/src/types.dart	52.63% <ø> (ø)
lib/src/binary_codec.dart	96.65% <93.54%> (ø)

... and 1 file with indirect coverage changes

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