v0.18.0: Implementation of generic newtypes

[brycx]

closes #249 #274

First and by far the largest set of breaking changes for version 0.18.0. A re-design of the newtypes used across the library has been a long time coming, at least since #249 and work started in #274. The work behind this new interface has largely been done by @vlmutolo in #274 and this PR uses the same interface, with a few tweaks.

The new interface includes:

• Generalized structs Public<T: TypeSpec> and Secret<T: TypeSpec> where the public trait TypeSpec is implemented for zero-sized structs to contextualize the uses across different primitives/schemes.
• Public<T> and Secret<T> can additionally implement GenerateSecret/GeneratePublic to indicate that either one can be generated using a CSPRNG, within a given context. A major change here is also that all randomness-generation has become fallible. There is no Default that panics and there is no generate() that panics. This has been motivated by an effort to use it without hiding possible panic!()s and making auditing hereof easier.

The generalized approach offers much more flexibility for current and future types to be added. A good example is that the previous newtypes weren't usable to ML-KEM encapsulation/decapsulation keys due to additional checking requirements, etc. This generalized approach has allowed us to implement it for these as well. The result is also a more consistent API experience across different parts of the library.

Finally, removing the use of macros has reduced code-expansion from this, amount of duplicate tests being run (for newtypes having the same underlying primitive/sizes, etc.).

The amount of changes here justify a new "Discussion" for v0.18.0 that includes a migration guide.

[codecov]

Codecov Report

❌ Patch coverage is 99.23208% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.24%. Comparing base (a5c3085) to head (f7630bd).

Files with missing lines	Patch %	Lines
src/hazardous/kem/ml_kem/internal/mod.rs	96.37%	10 Missing ⚠️
src/high_level/pwhash.rs	92.85%	3 Missing ⚠️
tests/stream/mod.rs	50.00%	3 Missing ⚠️
src/hazardous/kem/ml_kem/mlkem1024.rs	98.59%	2 Missing ⚠️
src/hazardous/kem/ml_kem/mlkem512.rs	98.59%	2 Missing ⚠️
src/hazardous/kem/ml_kem/mlkem768.rs	98.59%	2 Missing ⚠️
src/hazardous/kem/xwing.rs	99.17%	2 Missing ⚠️
src/generics/data.rs	99.43%	1 Missing ⚠️
src/hazardous/aead/streaming.rs	98.41%	1 Missing ⚠️
src/hazardous/kem/x25519_hkdf_sha256.rs	97.36%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           v0.18.0     #615      +/-   ##
===========================================
+ Coverage    99.14%   99.24%   +0.09%     
===========================================
  Files          108      113       +5     
  Lines        19957    21243    +1286     
===========================================
+ Hits         19787    21082    +1295     
+ Misses         170      161       -9     

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