Replace i32 with u32 in build.rs

[jayvdb]

clippy says n and i are i32 and warns about the cast to usize

error: casting `i32` to `usize` may lose the sign of the value
  --> build.rs:32:32
   |
32 |         let mut crc = table[0][n as usize];
   |                                ^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_sign_loss

This calms down clippy about those, and a few other minor issues it raises.