[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.

platform/default/string_stdlib.cpp

@@ -1,10 +1,10 @@
 #include <mbgl/platform/platform.hpp>
-#include <mbgl/util/utf.hpp>
 #define NU_WITH_TOUPPER
 #define NU_WITH_TOLOWER
 #define NU_WITH_UTF8_WRITER
 #include <libnu/libnu.h>
 #include <cstring>
+#include <sstream>
 
 namespace mbgl { namespace platform {