[Rust] Serialize to File or generic `std::io::Write`

Hi!

From my understanding, the a flexbuffer has to be built using a `Builder`, which serializes to an internal `Vec<u8>`. Would it be possible to serialize to a file?

Generally give a type that implements `std::io::Write`. This is a problem to me because I'm serializing a struct that barely fits in memory, so the serialization buffer gives me an out of memory.

Thank you