Fix LightGBM models locale sensitivity and improve R/W performance.

When Java is used, the default C++ locale is broken. This is true for
Java providers that use the C API or even Python models that require JEP.

This patch solves that issue making the model reads/writes insensitive
to such settings.
To achieve it, within the model read/write codebase:
 - C++ streams are imbued with the classic locale
 - Calls to functions that are dependent on the locale are replaced
 - The default locale is not changed!

This approach means:
 - The user's locale is never tampered with, avoiding issues such as
    microsoft#2979 with the previous
    approach microsoft#2891
 - Datasets can still be read according the user's locale
 - The model file has a single format independent of locale

Changes:
 - Add CommonC namespace which provides faster locale-independent versions of Common's methods
 - Model code makes conversions through CommonC
 - Cleanup unused Common methods
 - Performance improvements. Use fast libraries for locale-agnostic conversion:
   - value->string: https://github.com/fmtlib/fmt
   - string->double: https://github.com/lemire/fast_double_parser (10x
      faster double parsing according to their benchmark)

Bugfixes:
 - microsoft#2500
 - microsoft#2890
 - ninia/jep#205 (as it is related to LGBM as well)

.gitmodules

@@ -1,3 +1,9 @@
 [submodule "include/boost/compute"]
 	path = compute
 	url = https://github.com/boostorg/compute
+[submodule "external_libs/fmt"]
+	path = external_libs/fmt
+	url = https://github.com/fmtlib/fmt.git
+[submodule "external_libs/fast_double_parser"]
+	path = external_libs/fast_double_parser
+	url = https://github.com/lemire/fast_double_parser.git

CMakeLists.txt

@@ -444,6 +444,11 @@ if(__BUILD_FOR_R)
   endif(MSVC)
 endif(__BUILD_FOR_R)
 
+# fmtlib/fmt
+add_subdirectory(external_libs/fmt)
+TARGET_LINK_LIBRARIES(lightgbm PUBLIC fmt::fmt)
+TARGET_LINK_LIBRARIES(_lightgbm PUBLIC fmt::fmt)
+
 install(TARGETS lightgbm _lightgbm
         RUNTIME DESTINATION ${CMAKE_INSTALL_PREFIX}/bin
         LIBRARY DESTINATION ${CMAKE_INSTALL_PREFIX}/lib