[HW] Pass to flatten !hw.struct types

Nobody has claimed this yet.

As discussed in the ODM on 5/18/22. The idea is to transform
```mlir
hw.module @foo(%in: !hw.struct) -> (out: !hw.struct) {
%0 = hw.struct_extract %in["a"] : !hw.struct
%1 = hw.struct_extract %in["b"] : !hw.struct
%2 = hw.struct_create (%1, %0) : !hw.struct
hw.output %2 : !hw.struct
}
```
to
```mlir
hw.module @foo_flat(%in_a: i8, %in_b: i16) -> (out_c: i16, out_d: i8) {
hw.output %in_b, %in_a : i16, i8
}
```

Use the two MLIR examples in the issue as the starting specification for the flattening pass. Trace how hw.struct types, hw.struct_extract, and hw.struct_create are represented and handled, then verify that the pass produces flattened module inputs and outputs with the shown ordering and equivalent values.

Written by the indexing model from the issue text.