Improve error message for pointer casts

[khyperia]

when running on #149:

  error: Cannot cast between pointer types
     --> /home/khyperia/.cargo/git/checkouts/glam-rs-c6c9bb5297445777/0c13d3a/src/f32/vec3a.rs:867:20
      |
  867 |         unsafe { &*(self as *const Vec3A as *const [f32; 3]) }
      |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: from: *{Function} struct f32::vec3a::Vec3A { 0: struct f32::vec3::Vec3 { 0: f32, 1: f32, 2: f32 } }
      = note: to: *{Function} [f32; 3]

[repi]

very nice!