Re: range_agg

I took the liberty of rebasing this series on top of recent branch
master. The first four are mostly Paul's originals, except for conflict
fixes; the rest are changes I'm proposing as I go along figuring out the
whole thing. (I would post just my proposed changes, if it weren't for
the rebasing; apologies for the messiness.)

I am not convinced that adding TYPTYPE_MULTIRANGE is really necessary.
Why can't we just treat those types as TYPTYPE_RANGE and distinguish
them using TYPCATEGORY_MULTIRANGE? That's what we do for arrays. I'll
try to do that next.

I think the algorithm for coming up with the multirange name is
suboptimal. It works fine with the name is short enough that we can add
a few extra letters, but otherwise the result look pretty silly. I
think we can still improve on that. I propose to make
makeUniqueTypeName accept a suffix, and truncate the letters that appear
*before* the suffix rather than truncating after it's been appended.

There's a number of ereport() calls that should become elog(); and a
bunch of others that should probably acquire errcode() and be
reformatted per our style.

Regarding Pavel's documentation markup issue,

> I am not sure how much is correct to use <literallayout class="monospaced">
> in doc. It is used for ranges, and multiranges, but no in other places

I looked at the generated PDF and the table looks pretty bad; the words
in those entries overlap the words in the cell to their right. But that
also happens with entries that do not use <literallayout class="x">!
See [1] for an example of the existing docs being badly formatted. The
docbook documentation [2] seems to suggest that what Paul used is the
appropriate way to do this.

Maybe a way is to make each entry have more than one row -- so the
example would appear below the other three fields in its own row, and
would be able to use the whole width of the table.

[1] https://twitter.com/alvherre/status/1205563468595781633
[2] https://tdg.docbook.org/tdg/5.1/literallayout.html

--
Álvaro Herrera https://www.2ndQuadrant.com/
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