Slightly improve imports in plane.rs

[FreezyLemon]

Two small changes:

• Remove use std::u32 and use std::usize (there is no reason to import these)
• Import std::mem::size_of directly instead of just std::mem (other files in the project do the same)

I think the only reason to use std::u32 nowadays would be using the deprecated constants? I think you get to use the integer primitives even when disabling the Rust prelude...

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0244b29) 66.52% compared to head (b3f7676) 66.52%.

Files	Patch %	Lines
src/plane.rs	33.33%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   66.52%   66.52%           
=======================================
  Files           4        4           
  Lines         923      923           
=======================================
  Hits          614      614           
  Misses        309      309           

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