Make Zeroization Optional

[achimcc]

This implements issue #315

Make zeroization optional via a zeroize feature flag

This PR makes the zeroize dependency optional behind a new zeroize Cargo
feature (enabled by default). This allows users who don't need automatic
memory zeroization to reduce their dependency footprint.

Changes

• Cargo.toml: Made zeroize an optional dependency with a dedicated
  dep:zeroize feature. Added zeroize?/alloc to safe_api for heap-allocated
  types.
• typedefs.rs: Added three helpers for conditional zeroization:
  • zeroize_wrap! macro — wraps a value in Zeroizing or returns it as-is
  • ZeroizeWrap type alias — resolves to Zeroizing or plain T
  • zeroize_call! macro — calls .zeroize() or is a no-op
• Drop impls: Gated all impl Drop blocks behind #[cfg(feature = "zeroize")]
• SHA2 Word trait: Replaced the hard Zeroize bound with a MaybeZeroize helper
  trait that is conditionally empty
• Function signatures: Replaced Zeroizing in internal APIs with
  ZeroizeWrap
• Direct .zeroize() calls: Replaced with zeroize_call!() across AEAD, KDF,
  KEM, and MAC modules

Testing

Compiles cleanly with both cargo check (default, zeroize enabled) and cargo
check --no-default-features --features safe_api (zeroize disabled).

[codecov]

Codecov Report

❌ Patch coverage is 98.73418% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.13%. Comparing base (b3bb8b0) to head (6d11d4b).

Files with missing lines	Patch %	Lines
src/hazardous/kdf/scrypt.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
- Coverage   99.13%   99.13%   -0.01%     
==========================================
  Files         108      108              
  Lines       19970    19972       +2     
==========================================
+ Hits        19798    19799       +1     
- Misses        172      173       +1     

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[brycx]

Thanks a lot for taking the time to submit a PR for this @achimcc!

I have a few comments but most of it should be pretty simple fixes. I like the overall approach so far.