refactor: Use axum for "mdbook serve"

[audunhalland]

This in my opinion brings mdbook's dependencies more in line with "the state of the art" of the Rust HTTP ecosystem.
It pulls in hyper v1 and uses tower-http for filesystem serving functions.

I've manually tested serve with the test_book:

• serving pages from the book works
• websocket-based auto-reload works
• 404 page works

I can't guarantee though that there are no regressions. I did not find integrations tests for the serve command.

[paolobarbolini]

FWIW this seems the right call, as warp is still on hyper v0.14 and looking at their repository it gets almost no maintenance.

[audunhalland]

rebased, squashed commits; upgraded to axum 0.8.1.

[szabgab]

I think the change raising msrv to 1.75 should be removed from the PR.
Especially as mdbook is now on 1.77 already.

[szabgab]

Thanks for the update.

I am not familiar with axum so I can't comment on the code, but I tried the PR locally and it works on Linux.

My only thought was that by replacing a small framework with a larger one we might make the executable much larger.
So I checked the size of the target/release/mdbook executable on Ubuntu Linux after generating it with the cargo build --locked --bin mdbook --release command:

• on master 14,955,672
• with axum 13,146,536

So somehow this change also reduces the size of the executable by some 13%. Any idea how and why?

[audunhalland]

Speculation:

• The http crate has been unified on version 1 (I see no other unification in the Cargo.lock diff)
• This change removes support for HTTP/2 as axum default features has been turned off (edit: actually after checking: http2 is not in the default feature set of axum)

[rustbot]

☔ The latest upstream changes (possibly 9822c2a) made this pull request unmergeable. Please resolve the merge conflicts.