VarUInt encoding is incorrect for sufficiently large u64s

[zslayton]

In the process of comparing the encoding primitives of Ion 1.0 and 1.1 I discovered a bug in 1.0's VarUInt::write_u64. This test fails:

    #[test]
    fn roundtrip_varuint() -> IonResult<()> {
        let mut buffer = Vec::new();
        VarUInt::write_u64(&mut buffer, u64::MAX)?;
        let (var_uint, _) = ImmutableBuffer::new(buffer.as_slice()).read_var_uint()?;
        let value = var_uint.value() as u64;
        assert_eq!(value, u64::MAX);
        Ok(())
    }

Output:

assertion `left == right` failed
  left: 9223372036854775807
  right: 18446744073709551615