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
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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(-)