Support memset on structs

[nazar-pc]

I decided to ask Claude to fix #312 and it turned out fairly simple.

Here is LLM-generated description of changes:

Implement memset_const_pattern and memset_dynamic_pattern for SpirvType::Adt by recursively building a fill pattern per field and composing them into a struct constant/composite, instead of hitting fatal!("memset on structs not implemented yet").

This unblocks crates that zero-init or byte-fill whole structs (e.g. core::mem::zeroed::<T>() on a struct type), such as the blake3 crate hitting this on self.buf = [0; BLOCK_LEN].

With this change I can add blake3 crate to dependencies and things continue to compile without issues, while previously I was hitting an error like this:

  error: memset on structs not implemented yet
     --> /home/nazar-pc/.cargo/git/checkouts/blake3-697b574540d368df/c7f0d21/src/lib.rs:550:17
      |
  550 |                 self.buf = [0; BLOCK_LEN];
      |                 ^^^^^^^^^^^^^^^^^^^^^^^^^

Fixes #312, closes #328