pgsql: Support tied Perl objects in plperl and associated contrib modul

Support tied Perl objects in plperl and associated contrib modules.

Up to now, if you passed a "tied" object to a PL/Perl operation,
it would most likely get treated as "undef", since we failed to
perform get magic on values we're passed. Tied hashes worked
partially, in that you could read out their keys, but the values
always read as undef.

To fix, use hv_iterval() not HeVAL() to fetch values from perl
hashes, and be careful to apply plperl_materialize_sv() before
testing SvOK() or SvROK() on any perl SV.

Add regression test cases exercising some typical usage scenarios
for tied objects.

Arguably this is a bug fix, but in view of the lack of field
complaints to date, let's treat it as a new feature instead.
The risk/reward ratio for back-patching isn't attractive.

Author: Andrey Rachitskiy <pl0h0yp1(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/569769.1786901901@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ccab6d36d0cbe554c380f69ade1598a41c06552a

Modified Files
--------------
contrib/hstore_plperl/expected/hstore_plperl.out | 25 +++++
contrib/hstore_plperl/hstore_plperl.c | 10 +-
contrib/hstore_plperl/sql/hstore_plperl.sql | 22 ++++
contrib/jsonb_plperl/expected/jsonb_plperl.out | 46 +++++++-
contrib/jsonb_plperl/jsonb_plperl.c | 4 +-
contrib/jsonb_plperl/sql/jsonb_plperl.sql | 39 +++++++
src/pl/plperl/GNUmakefile | 2 +-
src/pl/plperl/expected/plperl_tied.out | 132 +++++++++++++++++++++++
src/pl/plperl/meson.build | 1 +
src/pl/plperl/plperl.c | 44 ++++++--
src/pl/plperl/plperl.h | 10 ++
src/pl/plperl/sql/plperl_tied.sql | 113 +++++++++++++++++++
12 files changed, 431 insertions(+), 17 deletions(-)