Patch for SortSupport implementation on inet/cdir

Hi list,

I've attached a patch that implements SortSupport for the
inet/cidr types. It has the effect of typically reducing
the time taken to sort these types by ~50-60% (as measured
by `SELECT COUNT(DISTINCT ...)` which will carry over to
common operations like index creation, `ORDER BY`, and
`DISTINCT`.

As with other SortSupport implementations, this one
proposes an abbreviated key design that packs in as much
sorting-relevant information into the available datum as
possible while remaining faithful to the existing sorting
rules for the types. The key format depends on datum size
and whether working with IPv4 or IPv6. In most cases that
involves storing as many netmask bits as we can fit, but we
can do something a little more complete with IPv4 on 64-bit
— because inet data is small and the datum is large, we
can store enough information for a successful key-only
comparison in the majority of cases. Precise details
including background and bit-level diagrams are included in
the comments of the attached patch.

I've tried to take a variety of steps to build confidence
that the proposed SortSupport-based sort is correct:

1. It passes the existing inet regression suite (which was
pretty complete already).

2. I added a few new test cases the suite, specifically
trying to target edge cases like the minimums and
maximums of each abbreviated key bit "boundary". The new
cases were run against the master branch to double-check
that they're right.

3. I've added a variety of assertions throughout the
implementation to make sure that each step is seeing
inputs with expected parameters, and only manipulates
the bits that it's supposed to be manipulating.

4. I built large random data sets (10M rows) and sorted
them against a development build to try and trigger the
aforementioned assertions.

5. I built an index on 10M values and ran amcheck against
it.

6. I wrote some unit tests to verify that the bit-level
representation of the new abbreviated keys are indeed
what we expect. They're available here [3]. I didn't
include them in the patch because I couldn't find an
easy way to produce an expected `.out` file for a 32-bit
machine (experiments building with `setarch` didn't
succeed). They might be overkill anyway. I can continue
to pursue getting them working if reviewers think they'd
be useful.

My benchmarking methodology and script is available here
[1], and involves gathering statistics for 100
`count(distinct ...)` queries at various data sizes. I've
saved the results I got on my machine here [2].

Hat tip to Peter Geoghegan for advising on an appropriate
abbreviated key design and helping answer many questions
along the way.

Brandur