Create contrib/bool_plperl to provide a bool transform for PL/Perl[U].

plperl's default handling of bool arguments or results is not terribly
satisfactory, since Perl doesn't consider the string 'f' to be false.
Ideally we'd just fix that, but the backwards-compatibility hazard
would be substantial.  Instead, build a TRANSFORM module that can
be optionally applied to provide saner semantics.

Perhaps usefully, this is also about the minimum possible skeletal
example of a plperl transform module; so it might be a better starting
point for user-written transform modules than hstore_plperl or
jsonb_plperl.

Ivan Panchenko

Discussion: https://postgr.es/m/1583013317.881182688@f390.i.mail.ru

contrib/bool_plperl/bool_plperl--1.0.sql

@@ -0,0 +1,19 @@
+/* contrib/bool_plperl/bool_plperl--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION bool_plperl" to load this file. \quit
+
+CREATE FUNCTION bool_to_plperl(val internal) RETURNS internal
+LANGUAGE C STRICT IMMUTABLE
+AS 'MODULE_PATHNAME';
+
+CREATE FUNCTION plperl_to_bool(val internal) RETURNS bool
+LANGUAGE C STRICT IMMUTABLE
+AS 'MODULE_PATHNAME';
+
+CREATE TRANSFORM FOR bool LANGUAGE plperl (
+    FROM SQL WITH FUNCTION bool_to_plperl(internal),
+    TO SQL WITH FUNCTION plperl_to_bool(internal)
+);
+
+COMMENT ON TRANSFORM FOR bool LANGUAGE plperl IS 'transform between bool and Perl';