Upgrade to edition 2024

[youknowone]

for this project, edition affects a lot to cargo test speed

edition 2021

time cargo test > /dev/null 
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running unittests src/lib.rs (target/debug/deps/vecmap-e7467bed5d40b8ad)
   Doc-tests vecmap
cargo test > /dev/null  14.88s user 7.99s system 45% cpu 49.809 total

edition 2024

time cargo test > /dev/null
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running unittests src/lib.rs (target/debug/deps/vecmap-e7467bed5d40b8ad)
   Doc-tests vecmap
cargo test > /dev/null  0.77s user 0.27s system 98% cpu 1.058 total

[martinohmann]

Thanks!