Remove `tuntap` crate

[n0toose]

I can mention more details later, but the tuntap crate is basically the sole reason why cross-compiling won't work anymore after #1120.

This issue is visible when running cargo build --target=x86_64-unknown-linux-musl, but I also came across problems involving FFI when trying to port to Linux aarch64 (both GNU and musl as well).