Speed up ICU case conversion by using ucasemap_utf8To*()

Hi,

Jeff pointed out to me that the case conversion functions in ICU have
UTF-8 specific versions which means we can call those directly if the
database encoding is UTF-8 and skip having to convert to and from UChar.

Since most people today run their databases in UTF-8 I think this
optimization is worth it and when measuring on short to medium length
strings I got a 15-20% speed up. It is still slower than glibc in my
benchmarks but the gap is smaller now.

SELECT count(upper) FROM (SELECT upper(('Kålhuvud ' || i) COLLATE
"sv-SE-x-icu") FROM generate_series(1, 1000000) i);

master: ~540 ms
Patched: ~460 ms
glibc: ~410 ms

I have also attached a clean up patch for the non-UTF-8 code paths. I
thought about doing the same for the new UTF-8 code paths but it turned
out to be a bit messy due to different function signatures for
ucasemap_utf8ToUpper() and ucasemap_utf8ToLower() vs ucasemap_utf8ToTitle().

Andreas