Use a u32 for MCTPTransportHeader storage

[mkj]

This saves ~800 bytes of code size from a current project (cortex M7, opt "s" or "z"), and uses a lot fewer cycles.
I'm pushing this as a draft PR, if you like the approach I could change the other bitfields similarly.

This does change the API a bit, instead of hdr.0 you use hdr.to_bytes(), and the MCTPTransportHeader no longer has the type parameter MCTPTransportHeader<[u8; 4]>.

This compiles to much more efficient code in the bitfield operations.

The code I'm testing with is fairly simple, setting a few headers similar to:

        let mut header = MCTPTransportHeader::new(1);
        header.set_dest_endpoint_id(dest);
        header.set_source_endpoint_id(src);
        header.set_pkt_seq(seq);
        if self.first {
            header.set_som(first);
        }
        header.set_msg_tag(tag);
        header.set_to(is_owner);

[alistair23]

I'm fine with the change. Saving code size is great!