It no longer has any users. If we need it later, we can add it back.
This saves one pointer of memory from `sizeof(upb_Arena)`.
Also, we now allow fuses if if the block allocators differ. This is made possible by cl/520144430, but was not take advantage of in that CL.
PiperOrigin-RevId: 520174588
This fixes an MSAN warning of the form:
```
WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x7fc964f2a597 in arena_findroot third_party/upb/upb/mem/arena.c:64:3
#1 0x7fc964f2a597 in upb_Arena_Free third_party/upb/upb/mem/arena.c:211:7
#2 0x7fc9d2af0028 in std::__msan::unique_ptr<upb_Arena, void (*)(upb_Arena*)>::~unique_ptr() third_party/crosstool/v18/stable/toolchain/bin/../include/c++/v1/__memory/unique_ptr.h:277:7
#3 0x7fc9d2aef7a2 in ~Arena third_party/upb/upb/upb.hpp:70:7
#4 0x7fc9d2aef7a2 in ~InlinedArena third_party/upb/upb/upb.hpp:97:7
#5 0x7fc9d2aef7a2 in Cpp_InlinedArena2_Test::TestBody() third_party/upb/upb/test/test_cpp.cc:187:1
#6 0x7fc97da78a57 in testing::Test::Run() third_party/googletest/googletest/src/gtest.cc:2695:5
#7 0x7fc97da7a3e8 in testing::TestInfo::Run() third_party/googletest/googletest/src/gtest.cc:2844:11
#8 0x7fc97da7b897 in testing::TestSuite::Run() third_party/googletest/googletest/src/gtest.cc:3003:30
#9 0x7fc97daa5136 in testing::internal::UnitTestImpl::RunAllTests() third_party/googletest/googletest/src/gtest.cc:5899:44
#10 0x7fc97daa455c in HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> third_party/googletest/googletest/src/gtest.cc
#11 0x7fc97daa455c in testing::UnitTest::Run() third_party/googletest/googletest/src/gtest.cc:5464:10
#12 0x562a7fb876f0 in RUN_ALL_TESTS third_party/googletest/googletest/include/gtest/gtest.h:2329:73
#13 0x562a7fb876f0 in main testing/base/internal/gunit_main.cc:86:10
#14 0x7fc9ba9b7632 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x61632) (BuildId: 280088eab084c30a3992a9bce5c35b44)
#15 0x562a7fafdbe9 in _start /build/work/ab393f4ac612f9027aae6b1a7226027ba2a2/google3/blaze-out/k8-opt/bin/third_party/grte/v5_src/grte-scratch/BUILD/src/csu/../sysdeps/x86_64/start.S:120
Member fields were destroyed
#0 0x562a7fb0b13d in __sanitizer_dtor_callback_fields third_party/llvm/llvm-project/compiler-rt/lib/msan/msan_interceptors.cpp:960:5
#1 0x7fc9d2aef79a in ~InlinedArena third_party/upb/upb/upb.hpp:105:8
#2 0x7fc9d2aef79a in ~InlinedArena third_party/upb/upb/upb.hpp:97:7
#3 0x7fc9d2aef79a in Cpp_InlinedArena2_Test::TestBody() third_party/upb/upb/test/test_cpp.cc:187:1
#4 0x7fc97da78a57 in testing::Test::Run() third_party/googletest/googletest/src/gtest.cc:2695:5
#5 0x7fc97da7a3e8 in testing::TestInfo::Run() third_party/googletest/googletest/src/gtest.cc:2844:11
#6 0x7fc97da7b897 in testing::TestSuite::Run() third_party/googletest/googletest/src/gtest.cc:3003:30
#7 0x7fc97daa5136 in testing::internal::UnitTestImpl::RunAllTests() third_party/googletest/googletest/src/gtest.cc:5899:44
#8 0x7fc97daa455c in HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> third_party/googletest/googletest/src/gtest.cc
#9 0x7fc97daa455c in testing::UnitTest::Run() third_party/googletest/googletest/src/gtest.cc:5464:10
#10 0x562a7fb876f0 in RUN_ALL_TESTS third_party/googletest/googletest/include/gtest/gtest.h:2329:73
#11 0x562a7fb876f0 in main testing/base/internal/gunit_main.cc:86:10
#12 0x7fc9ba9b7632 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x61632) (BuildId: 280088eab084c30a3992a9bce5c35b44)
#13 0x562a7fafdbe9 in _start /build/work/ab393f4ac612f9027aae6b1a7226027ba2a2/google3/blaze-out/k8-opt/bin/third_party/grte/v5_src/grte-scratch/BUILD/src/csu/../sysdeps/x86_64/start.S:120
```
PiperOrigin-RevId: 511849224
_upb_Arena_FastMalloc() has been directly inlined
Arena::allocator() has been removed
mem_block is now _upb_MemBlock
some calls to upb_Arena_Alloc() (which is going away) have been changed to use upb_Arena_Malloc() instead
PiperOrigin-RevId: 489874377
This makes both the C (.h) and C++ (.hpp) files read nicer
and keeps the core of upb C-only.
Existing users of the C++ wrappers will have to add manual
#includes of the .hpp files.