Possible unaligned write with MutableBuffer::push

[jhorstmann]

The following test fails when run under miri:

    #[test]
    fn mutable_push_unaligned_u64() {
        let mut buf = MutableBuffer::new(16);
        buf.push(1_u8);
        buf.push(1_u64);
        assert_eq!(9, buf.len());
        assert_eq!(&[1u8, 1u8, 0, 0, 0, 0, 0, 0, 0], buf.as_slice());
    }

The code uses std::ptr::write to append the u64, regardless of the current alignment. Replacing that with write_unaligned would probably work. The type parameter is constrained by the ToByteSlice trait, but it seems the trait is not actually used. Not sure whether this is expected. It seems ToByteSlice is more of a marker trait here that indicates that the contents can be transmuted to a slice of bytes.