Now `cxx` doesn't support `rust::Vec<rust::Vec<T>>`

[moshimo2019]

When I used rust::Vec<rust::Vec<T>> in c++, I got the following error:

  = note: bazel-out/k8-opt/bin/libbridge.a(lib.rs.o):lib.rs.cc:function cxxbridge1$check: error: undefined reference to 'rust::cxxbridge1::Vec<rust::cxxbridge1::Vec<rust::cxxbridge1::String> >::Vec()'
          bazel-out/k8-opt/bin/libbridge.a(lib.rs.o):lib.rs.cc:function cxxbridge1$check: error: undefined reference to 'rust::cxxbridge1::Vec<rust::cxxbridge1::Vec<rust::cxxbridge1::String> >::drop()'
          collect2: error: ld returned 1 exit status