Add new CREATE CONVERSION/DROP CONVERSION command.

This is the first cut toward CREATE CONVERSION/DROP CONVERSION implementaion.
The commands can now add/remove tuples to the new pg_conversion system
catalog, but that's all. Still need work to make them actually working.
Documentations, regression tests also need work.

src/test/regress/expected/sanity_check.out

@@ -38,6 +38,7 @@ SELECT relname, relhasindex
  pg_attrdef          | t
  pg_attribute        | t
  pg_class            | t
+ pg_conversion       | t
  pg_database         | t
  pg_description      | t
  pg_group            | t
@@ -60,5 +61,5 @@ SELECT relname, relhasindex
  shighway            | t
  tenk1               | t
  tenk2               | t
-(50 rows)
+(51 rows)