Add dgram crate

[evanrittenhouse]

Overview

This PR adds a new crate for abstracting away direct interactions with cmsgs. It currently only contains implementations for Linux, but we can add multi-platform support when required.

Tasks

• Review TODOs
• Ensure parity with existing error returns from e.g. quiche/apps/sendto
• Check syscall tests again after API finalization
• More unit tests
• Redo test plan once PR is published
• Test plan on master as a control group
• Unify Linux/non-Linux API signatures
• Test async

Testing

Unit tests exist for basic functionality, but should probably be more robust.

To check the actual GSO/GRO syscall counts, I'm using quiche-server and quiche-client. The general test plan is to make 30 requests to quiche-server, returning a 1MB body each time to trigger batching (e.g. modifying the Not Found! response). I used the following command to grab the syscall counts:

strace -p $(pgrep quiche-server) \
-o $(pwd)/syscall-counts/server-{master,dgram}{-gsogro}.txt \
-e trace="sendmsg,sendto,recvmsg,recvfrom"

and for sending the requests:

for i in (seq 0 30)
     cargo run --bin quiche-client -- https://127.0.0.1:8085 --no-verify
end

The syscall counts are as follows:
Server
No GSO/GRO

…uiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly took 1m26s
❯ rg -c "sendto" -g "syscall-counts/server-dgram.txt"
syscall-counts/server-dgram.txt:24025

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "sendmsg" -g "syscall-counts/server-dgram.txt"

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "recvfrom" -g "syscall-counts/server-dgram.txt"

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "recvmsg" -g "syscall-counts/server-dgram.txt"
syscall-counts/server-dgram.txt:26047

GSO/GRO

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "sendto" -g syscall-counts/server-dgram-gsogro.txt
syscall-counts/server-dgram-gsogro.txt:62

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "sendmsg" -g syscall-counts/server-dgram-gsogro.txt
syscall-counts/server-dgram-gsogro.txt:2542

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "recvfrom" -g syscall-counts/server-dgram-gsogro.txt

…debian in quiche on  evanrittenhouse/dgram [ ⇡1 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "recvmsg" -g syscall-counts/server-dgram-gsogro.txt
syscall-counts/server-dgram-gsogro.txt:5404

And on master, as a control:
No GSO

…an in quiche on  evanrittenhouse/mio_retry [ !2 ?7 *1 ] via Rust v1.79.0-nightly
❮ rg -c "sendto" -g syscall-counts/server-master.txt
syscall-counts/server-master.txt:19781

…an in quiche on  evanrittenhouse/mio_retry [ !2 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "sendmsg" -g syscall-counts/server-master.txt

GSO

…an in quiche on  evanrittenhouse/mio_retry [ !2 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "sendto" -g syscall-counts/server-master-gso.txt
syscall-counts/server-master-gso.txt:62

…an in quiche on  evanrittenhouse/mio_retry [ !2 ?7 *1 ] via Rust v1.79.0-nightly
❯ rg -c "sendmsg" -g syscall-counts/server-master-gso.txt
syscall-counts/server-master-gso.txt:2542