Support encoding/decoding of vararrays of any type

[leighmcculloch]

It should be possible to encode/decode vararrays of any type, such as doing the equivalent as:

use stellar_xdr::curr::{DepthLimitedRead, DiagnosticEvent, ReadXdr, VecM};

fn main() {
    let events: VecM<DiagnosticEvent> =
        VecM::read_xdr_base64_to_end(&mut DepthLimitedRead::new(std::io::stdin(), 500)).unwrap();

    serde_json::to_writer_pretty(std::io::stdout(), &events).unwrap();
}