Use analyzed expression trees for constraint construction.

[arssher]

Initially I planned this as an easy change which would

• Allow to use weird types such as bytea as hash partitioning key --
  currently we can't do that because hashvarlena declared as accepting
  'internal' type, and analyzer can't pass bytea to it.
• Make everything a bit simpler, because expression is analyzed anyway
  (cook_partitioning_expression), so a bit of code unification was expected.

Eventually it turned out to be much more cumbersome for various reasons (e.g.
analyzed trees should be adapted for childs due to dropped columns), so I am not
even sure it is worthwhile anymore.

Also, micro bug fix at partition_filter.c

[codecov-io]

Codecov Report

Merging #163 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #163   +/-   ##
=======================================
  Coverage   91.74%   91.74%           
=======================================
  Files          43       43           
  Lines        6384     6384           
=======================================
  Hits         5857     5857           
  Misses        527      527

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[arssher]

Suspended.