feat: add SoAble derive support for enums

   Implement struct-of-arrays transformation for enums with discriminant-based
   storage. Enums are decomposed into a separate discriminant array and union
   arrays for each field position.

   Features:
   - Generate discriminant-only enum with preserved explicit values
   - Create union types for each field position across variants
   - Support named and unnamed variant fields
   - Support unit variants and explicit discriminant expressions
   - Preserve enum visibility (pub, pub(crate), private)

   Safety:
   - Discriminant fields kept private in helper structs
   - Safe get_discriminant() accessor methods
   - Unsafe get_discriminant_mut() requires caller to maintain invariants
   - Prevents unsound discriminant/union field mismatches

   Tests added for discriminant access, named fields, and visibility.

Author: Reinsdyret

soavec_derive/src/lib.rs

@@ -2,6 +2,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
+pub(crate) mod soa_enum;
+pub(crate) mod soa_struct;
 mod soable;
 
 use proc_macro::TokenStream;