Rework code to determine partition pruning procedure

Rework code to determine partition pruning procedure

Amit Langote reported that partition prune was unable to work with
arrays, enums, etc, which led him to research the appropriate way to
match query clauses to partition keys: instead of searching for an exact
match of the expression's type, it is better to rely on the fact that
the expression qual has already been resolved to a specific operator,
and that the partition key is linked to a specific operator family.
With that info, it's possible to figure out the strategy and comparison
function to use for the pruning clause in a manner that works reliably
for pseudo-types also.

Include new test cases that demonstrate pruning where pseudotypes are
involved.

Author: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/2b02f1e9-9812-9c41-972d-517bdc0f815d@lab.ntt.co.jp