Implement UDP

[jastintime]

start off UDP header.

questions:
should we use a size_t or something similar or u64 everything
how do we want to model our general networking code(plan9 style would be sick 🔥 )

useful UDP links:
https://www.rfc-editor.org/rfc/rfc768

Notes so I don't forget:
UDP and TCP checksum is the same, can be brought out to a common file

[remexre]

should we use a size_t or something similar or u64 everything

if it's the size of a thing, usize (we don't have a literal equiv of ptrdiff_t, but we have isize ig)

how do we want to model our general networking code(plan9 style would be sick 🔥 )

you should do what you think is best lol

[jastintime]

TODO: fix CI format complaints ( requires make format on kieran branch first)

TODO: header guards on my headers

[jastintime]

good for review on Wednesday, needs some squashing, moving around and testing but core functionality is there.

[remexre]

gcc magic COUNTED_BY lets you say that len is the length of the buffer

[jastintime]

https://gcc.gnu.org/onlinedocs/gcc/Common-Attributes.html#index-counted_005fby

this seems to apply only to struct fields.

u64 udp_send_datagram(u16 src_port, u16 dst_port, struct ip_address src_addr,
                      struct ip_address dst_addr, [[gnu::counted_by (len)]] u8 *data, usize len);

results in

udp.h:53:81: error: 'counted_by' attribute is not allowed for a non-field declaration 'data'

[jastintime]

this now depends on #65 which depends on #73 being merged.

will fail CI until those get merged, should be good for looking over though.

7ce6d6b will be removed and udp added to net/include.mak once #73 gets merged.