spirv-fuzz: Transformation to flatten struct fields

If one struct contains another struct as a field, then we can probably flatten the latter and include all its fields into the outer struct.

Situations to consider:
- What if the inner struct is referenced directly in the module (e.g. extracted from the struct). It might be possible to apply the transformation in that case but it might not be worth it.

The issue names no files, tests, or entry points, so begin by locating spirv-fuzz transformations and existing struct-handling tests. Use the nested-struct case and the directly referenced inner-struct case as the scenarios to clarify, then confirm the intended behavior before implementation.

Written by the indexing model from the issue text.