Last call for comments: fmgr rewrite [LONG]

I attach the current draft of my document explaining the upcoming
function manager interface changes. This has been modified from the
previously circulated version on the basis of comments from Jan and
others. There is also a preliminary fmgr.h showing actual code for
the proposed interface macros.

Current implementation status: the core fmgr routines have been
rewritten and tested, also the executor's function-call code and the
function handler routines for the three PL languages. I haven't yet
changed over any pg_proc entries to new-style except the PL handlers.
However, passing nulls into and out of SQL, plpgsql, and plperl
functions works properly now, and it would work in pltcl if pltcl had a
convention for distinguishing nulls from empty strings (Jan, do you want
to do something about that?). I still need to change trigger handling
so we can get rid of the CurrentTriggerData global variable, and then
can start working on updating individual function routines to the new
conventions. I will probably not start on that work until after we make
the 7.1 branch and I can commit what I have.

regards, tom lane