Trivial fns allowing control if a type is trivially encoded/decoded

[xunilrj]

Description

Continuation of #7488.

This PR introduces the option for each type to control its own is_trivial.

Primitive data types have direct implementation (only decoding bool is false, as we need to guarantee its value is zero or one).

Structs, tuples, arrays etc... are trivial if their runtime representation and "inner types" allow for trivial encoding or decoding.

Enums are more complicated as we need to control that the enum tag is valid; they are not trivial.

String arrays are not trivially encoded/decoded for backwards compatibility with "encoding v0" that needs padding in some cases.

Some tests are showing a small regression (6 gas), which comes from the introduced "if" not being optimised away. I want to solve this in another PR, where the "if" condition will come from a const, and IR generation will guarantee that only one branch will ever be emitted.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

---

Note

Add per-type trivial encode/decode flags and use raw_ptr fast paths in ABI (encode/decode) with supporting changes to codec, String/Vec, and tests.

• std/codec:
  • Introduce AbiEncode::is_encode_trivial and AbiDecode::is_decode_trivial; add helpers is_encode_trivial/is_decode_trivial.
  • Switch encode/abi_decode to branch on triviality; add decode_from_raw_ptr and raw-ptr-based decode_* helpers.
  • Make BufferReader parameter accessors return raw_ptr; minor refactors to use raw ptr copies.
  • Implement triviality for primitives, arrays/tuples (conditional), str[strN] non-trivial, enums non-trivial.
• std/string & std/vec: implement trivial encode/decode markers (non-trivial) and adapt codec impls.
• Tests/fixtures:
  • Update snapshots (IR/ASM/bytecode/gas), JSON ABI offsets, contract/predicate IDs, and logging expectations to match new encoding paths.

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[codspeed-hq]

CodSpeed Performance Report

Merging #7501 will degrade performances by 13.21%

_{Comparing xunilrj/trivial-fns-codec-traits (dddd944) with master (54e0169)}

Summary

❌ 1 regression
✅ 24 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	compile	5.3 s	6.1 s	-13.21%

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[JoshuaBatty]

-13.24% compile time regression is a bit unfortunate. Any way we can get this number down a bit?