[core] unify UTF-8/16 conversion to <boost/locale/encoding_utf.hpp>

platform/default/utf.cpp

@@ -1,13 +1,16 @@
 #include <mbgl/util/utf.hpp>
 
-#include <locale>
-#include <codecvt>
+#include <boost/locale/encoding_utf.hpp>
 
 namespace mbgl {
 namespace util {
 
-std::u16string utf8_to_utf16::convert(const std::string& utf8) {
-    return std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t>().from_bytes(utf8);
+std::u16string convertUTF8ToUTF16(const std::string& str) {
+    return boost::locale::conv::utf_to_utf<char16_t>(str);
+}
+
+std::string convertUTF16ToUTF8(const std::u16string& str) {
+    return boost::locale::conv::utf_to_utf<char>(str);
 }
 
 } // namespace util