Don't sort documents twice in SortFieldWriter#Refill

The doc_ids are already sorted in S_lazy_init_sorted_ids. We only have
to make sure that S_lazy_init_sorted_ids uses the doc_id as secondary
sort key.
Make SortFieldWriter#Refill obey the memory limit

The old logic was broken.
Initialize SortFieldWriter#run_tick to 1

Make sure we never use a run_tick of 0.
Free sorted_ids in SortFieldWriter a little earlier
Use counting sort to sort doc_ids in SortFieldWriter#Refill

Since we already have the ordinals for each doc_id, we can use a
counting sort. This uses a temporary array of size run_cardinality but
runs in linear time.
Convert t/221-sort_writer.t to C

Original commit by Nick Wellnhofer, amended by Marvin Humphrey to
replace "cnick" with "nickname".
Port t/015-sort_external.t to C

Original commit by Nick Wellnhofer, updated by Marvin Humphrey for
changes to the SortExternal API.
Port t/028-sortexrun.t to C

Original commit by Nick Wellnhofer, amended by Marvin Humphrey for
changes to the SortExternal API.
This closes #5.

(This commit is a test of Apache's Github integration and attempts to
close the pull request at <https://github.com/apache/lucy/pull/5>).
Eliminate NoCloneHash; use a plain Hash instead.
Track memory consumption in a Counter.

Anticipating the elimination of MemoryPool, duplicate tracking of memory
consumption in a simple "Counter" object.
Make SFWriterElem an ordinary object.

Stop allocating from MemoryPool.
Use an ordinary Hash for uniquing values.

Stop using a ZombieKeyedHash, with values allocated from a MemoryPool,
and instead use ordinary String, etc.
Eliminate MemoryPool from SortFieldWriter.

SortFieldWriter used to allocate values from the MemoryPool, which was
also used to track memory usage.  Now it uses ordinary objects and
memory consumption is tracked in a simple Counter, so the MemoryPool is
no longer needed.
Remove now-unused ZombieKeyedHash.
Remove test for VTable.
Remove unused var.

This commit will be consolidated into another before merging to master.
Improve mem tracking.

Cache size of value object.  Account for size of SFWriterElem.  Handle
blob fields better.
Cease unique-ifying values in SortFieldWriter.
Rename Clownfish::VTable to Clownfish::Class