speed up verifying UTF-8

For v10, I've split the patch up into two parts. 0001 uses pure C
everywhere. This is much smaller and easier to review, and gets us the most
bang for the buck.

One concern Heikki raised upthread is that platforms with poor
unaligned-memory access will see a regression. We could easily add an
#ifdef to take care of that, but I haven't done so here.

To recap: On ascii-only input with storage taken out of the picture,
profiles of COPY FROM show a reduction from nealy 10% down to just over 1%.
In microbenchmarks found earlier in this thread, this works out to about 7
times faster. On multibyte/mixed input, 0001 is a bit faster, but not
really enough to make a difference in copy performance.

0002 adds the SSE4 implementation on x86-64, and is equally fast on all
input, at the cost of greater complexity.

To reflect the split, I've changed the thread subject and the commitfest
title.
-- 
John Naylor
EDB: http://www.enterprisedb.com