pgsql: Implement "ALTER EXTENSION ADD object".

Implement "ALTER EXTENSION ADD object".

This is an essential component of making the extension feature usable;
first because it's needed in the process of converting an existing
installation containing "loose" objects of an old contrib module into
the extension-based world, and second because we'll have to use it
in pg_dump --binary-upgrade, as per recent discussion.

Loosely based on part of Dimitri Fontaine's ALTER EXTENSION UPGRADE
patch.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/5bc178b89f3ab93fb3845a941769c212f5eeaf1a

Modified Files
--------------
doc/src/sgml/extend.sgml              |   12 +
doc/src/sgml/ref/alter_extension.sgml |  194 ++++++++++++++++---
src/backend/commands/extension.c      |   59 ++++++-
src/backend/nodes/copyfuncs.c         |   16 ++
src/backend/nodes/equalfuncs.c        |   14 ++
src/backend/parser/gram.y             |  186 +++++++++++++++++-
src/backend/tcop/utility.c            |  357 ++++++++++++---------------------
src/include/commands/extension.h      |    2 +
src/include/nodes/nodes.h             |    1 +
src/include/nodes/parsenodes.h        |   15 +-
10 files changed, 601 insertions(+), 255 deletions(-)