Fix union field access by representing unions as structs

[LegNeato]

When a union has multiple fields, represent it as a struct with all fields at offset 0 instead of just using the largest field. This allows pointer casts between union fields to work correctly by enabling the recover_access_chain_from_offset function to find valid access chains.

Fixes #241.

[LegNeato]

@simensgreen can you check this fix?

[simensgreen]

@LegNeato Ok, in this simple example it works, but in a more complicated scenario, it will not.

Now I can reproduce both cannot cast between pointer types and cannot memcpy dynamically sized data, you can check it out in my repo

Full error messages:

error: cannot cast between pointer types
         from `*struct DataMatrix { array1d: [f32; 25], array2d: [[f32; 5]; 5], data_array: [struct Data { a: f32, b: [f32; 3], c: f32 }; 5] }`
           to `*[[f32; 5]; 5]`
    --> shader/src/lib.rs:35:77
     |
  35 |     pub fn array2d_ref(&self) -> &[[f32; 5]; 5] { unsafe { &self.array2d } }
     |                                                                             ^
     |
  note: used from within `<shader::DataMatrix>::array2d_ref`
    --> shader/src/lib.rs:35:77
     |
  35 |     pub fn array2d_ref(&self) -> &[[f32; 5]; 5] { unsafe { &self.array2d } }
     |                                                                             ^
  note: called by `shader::main`
    --> shader/src/lib.rs:50:18
     |
  50 |     output[id] = input_matrices[id].array2d_ref()[0];
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: cannot memcpy dynamically sized data
  --> shader/src/lib.rs:34:54
   |
34 |     pub fn array2d(self) -> [[f32; 5]; 5] { unsafe { self.array2d } }
   |                                                      ^^^^^^^^^^^^
   |
note: used from within `<shader::DataMatrix>::array2d`
  --> shader/src/lib.rs:34:54
   |
34 |     pub fn array2d(self) -> [[f32; 5]; 5] { unsafe { self.array2d } }
   |                                                      ^^^^^^^^^^^^
note: called by `shader::main`
  --> shader/src/lib.rs:52:18
   |
52 |     output[id] = input_matrices[id].array2d()[0];
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Firestar99]

blocked by #270 due to adding a test

[LegNeato]

Ok, this code is a minefield...back to the drawingboard ha.