[core] Use UTF-16 instead of UTF-32 for label features to avoid extra conversions and reduce in-memory size.

Continue to use uint32 as glyph ID to maintain Glyph PBF, even though we're only using 16 bits of that uint32.
Use std::codecvt instead of boost::unicode_iterator for UTF8->UTF16 conversions.

src/mbgl/text/glyph.cpp

@@ -3,7 +3,7 @@
 namespace mbgl {
 
 // Note: this only works for the BMP
-GlyphRange getGlyphRange(char32_t glyph) {
+GlyphRange getGlyphRange(char16_t glyph) {
     unsigned start = (glyph/256) * 256;
     unsigned end = (start + 255);
     if (start > 65280) start = 65280;