Include missing headers

[MichaelChirico]

• cpp11.cpp: <cstddef> for ptrdiff_t
• vroom_write.cc: for <functional> std::cref

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Our tooling also recommends directly including the following cpp11 headers to src/cpp11.cpp, but I omitted that as it feels more stylistic:

• "cpp11/as.hpp" for as_cpp and as_sexp
• "cpp11/integers.hpp" for integers

Happy to update the patch to include those if desired.

[jennybc]

Thanks!