support u128 and i128

[nlewycky]

Serde conditionally supports 128-bit integers. Here's the relevant snippet from serde's build.rs:

    // 128-bit integers stabilized in Rust 1.26:
    // https://blog.rust-lang.org/2018/05/10/Rust-1.26.html
    //
    // Disabled on Emscripten targets as Emscripten doesn't
    // currently support integers larger than 64 bits.
    if minor >= 26 && !emscripten {
        println!("cargo:rustc-cfg=integer128");
    }

If we add the obvious implementations to de.rs and ser.rs then it works.