fix: Fix most clippy warnings

[leomeinel]

This fixes all clippy warnings except:

error: approximate value of `f{32, 64}::consts::SQRT_2` found
   --> tests/arena-triangulation.rs:163:9
    |
163 |         1.41421
    |         ^^^^^^^
    |
    = help: consider using the constant directly
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#approx_constant
    = note: `#[deny(clippy::approx_constant)]` on by default

error: approximate value of `f{32, 64}::consts::SQRT_2` found
  --> tests/arena-merged.rs:40:9
   |
40 |         1.41421
   |         ^^^^^^^
   |
   = help: consider using the constant directly
   = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#approx_constant
   = note: `#[deny(clippy::approx_constant)]` on by default

error: could not compile `polyanya` (test "arena-triangulation") due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: approximate value of `f{32, 64}::consts::SQRT_2` found
  --> tests/arena.rs:40:9
   |
40 |         1.41421
   |         ^^^^^^^
   |
   = help: consider using the constant directly
   = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#approx_constant
   = note: `#[deny(clippy::approx_constant)]` on by default

error: could not compile `polyanya` (test "arena-merged") due to 1 previous error
error: could not compile `polyanya` (test "arena") due to 1 previous error

I wasn't sure if you'd prefer adding an ignore or using the const here.

Most warnings have been fixed with:

cargo clippy --workspace --all-targets --all-features --fix

[leomeinel]

I did remove things like the _ in generated code (by clippy --fix) 1.012_345 since this is used nowhere else, I hope this is okay.