Modernize codebase

- Enhance CMakeLists.txt files.
- Move to Boost Test from Google Test to support pre-C++11 compilers.
- Add more configurations on CI matrix builds.
- Other minor fixes

fuzz/unpack_pack_fuzzer.cpp

@@ -1,14 +1,14 @@
 #include <msgpack.hpp>
 
-extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+extern "C" int FuzzerTestOneInput(const uint8_t *data, size_t size) {
   try {
     // NOTE(derwolfe): by default the limits are set at 2^32-1 length. I'm
     // setting these at far smaller values to avoid OOMs
     const int test_limit = 1000;
     msgpack::object_handle unpacked = msgpack::unpack(reinterpret_cast<const char *>(data),
                                                       size,
-                                                      nullptr,
-                                                      nullptr,
+                                                      MSGPACK_NULLPTR,
+                                                      MSGPACK_NULLPTR,
                                                       msgpack::unpack_limit(test_limit,
                                                                             test_limit,
                                                                             test_limit,