Extend GB18030 encoding conversion to cover full Unicode range.

Our previous code for GB18030 <-> UTF8 conversion only covered Unicode code
points up to U+FFFF, but the actual spec defines conversions for all code
points up to U+10FFFF.  That would be rather impractical as a lookup table,
but fortunately there is a simple algorithmic conversion between the
additional code points and the equivalent GB18030 byte patterns.  Make use
of the just-added callback facility in LocalToUtf/UtfToLocal to perform the
additional conversions.

Having created the infrastructure to do that, we can use the same code to
map certain linearly-related subranges of the Unicode space below U+FFFF,
allowing removal of the corresponding lookup table entries.  This more
than halves the lookup table size, which is a substantial savings;
utf8_and_gb18030.so drops from nearly a megabyte to about half that.

In support of doing that, replace ISO10646-GB18030.TXT with the data file
gb-18030-2000.xml (retrieved from
http://source.icu-project.org/repos/icu/data/trunk/charset/data/xml/ )
in which these subranges have been deleted from the simple lookup entries.

Per bug #12845 from Arjen Nienhuis.  The conversion code added here is
based on his proposed patch, though I whacked it around rather heavily.