Rust: Add read xdr to end fns

[leighmcculloch]

What

Add read xdr to end fns to the generated Rust code, and use them for base64 decoding.

Why

A lot of the time when reading from a stream into a value we want to read the entire stream into that one value, and for there to be extra data in the stream that is an error, because we don't expect there to be any extra data, and that data might be something we're discarding when we shouldn't be.

It's helpful to have functions that do this for us, so that the caller doesn't have to do this themselves.

The _to_end style functions are aligned with an existing pattern in the std::io::Read trait, where functions that end in _to_end read the stream all the way to the end. Although the function names do read a little odd. There might be a better name for this, but I haven't found it.