Support rust::* types in C++ headers

If we find a `rust::*` type in a C++ header [per this list](https://docs.rs/cxx/1.0.28/cxx/#builtin-types), include that in our generated `cxx` bindings.

In theory this is just a matter of contributing such types to `known_types.rs`. In practice it won't be that simple...

First step would be to add some tests to `integration_tests.rs`.

Start with the rust::* builtin-types list in the linked cxx documentation, then inspect known_types.rs and integration_tests.rs. Add integration tests covering the supported types in C++ headers, trace how known types become generated cxx bindings, and confirm the tests pass with those types included.

Written by the indexing model from the issue text.