[FFI] Introduce GlobalDef for function registration

[tqchen]

This PR introduces reflection::GlobalDef for function registration, which makes the global function registration API more closely aligned with the new reflection style.

We will send followup PRs to transition some of the existing mechanisms to the new one.

[tqchen]

New example

TVM_FFI_STATIC_INIT_BLOCK({
  namespace refl = tvm::ffi::reflection;
  refl::GlobalDef()
    .def_packed("ffi.Array", [](ffi::PackedArgs args, Any* ret) {
      *ret = Array<Any>(args.data(), args.data() + args.size());
    })
    .def("ffi.ArrayGetItem", [](const ffi::ArrayObj* n, int64_t i) -> Any { return n->at(i); })
    .def("ffi.ArraySize", [](const ffi::ArrayObj* n) -> int64_t {
      return static_cast<int64_t>(n->size());
    })
    .def_packed("ffi.Map", [](ffi::PackedArgs args, Any* ret) {
      TVM_FFI_ICHECK_EQ(args.size() % 2, 0);
      Map<Any, Any> data;
      for (int i = 0; i < args.size(); i += 2) {
        data.Set(args[i], args[i + 1]);
      }
      *ret = data;
    })
    .def("ffi.MapSize", [](const ffi::MapObj* n) -> int64_t {
      return static_cast<int64_t>(n->size());
    })
    .def("ffi.MapGetItem", [](const ffi::MapObj* n, const Any& k) -> Any { return n->at(k); })
    .def("ffi.MapCount", [](const ffi::MapObj* n, const Any& k) -> int64_t { return n->count(k); })
    .def("ffi.MapForwardIterFunctor", [](const ffi::MapObj* n) -> ffi::Function {
      return ffi::Function::FromTyped(MapForwardIterFunctor(n->begin(), n->end()));
    });
});