Reduce serde and iterator forwarding boilerplate

[jskoiz]

Summary

Collapses hand-copied forwarding methods into three small private macro_rules!, replacing ~115 near-identical method bodies. Net −638 src LOC (31,070 → 30,432) with no behavior change.

File	Change	LOC
src/with.rs	forward_singleton_deserialize! (both Deserializer impls, 48 methods) + forward_visit_scalars! (recursive Visitor's 20 scalar visits)	−602
src/ser.rs	serialize_value_from_scalars! across 5 Serializer impls (10 uniform numeric methods)	−252 / +114
src/ast.rs	mapping_projection_iter! for 7 Mapping iterator views	−222 / +94
src/event_de/source.rs	remove unused scalar_key_at / scalar_key_at_in	−18

What is intentionally left explicit

These are not boilerplate and stay hand-written:

• with.rs: the recursive visit_some / visit_seq / visit_map / visit_newtype_struct (real recursion) and the *AsEnum visitors.
• ser.rs: bool, i128, u128 keep per-serializer handling (lossless 128-bit narrowing in ValueSerializer, width preservation in PreserveNumberSerializer). &mut Serializer<W> is untouched.
• ast.rs: IntoIter (identity, no projection).

Verification

• cargo test: 909 passed, 0 failed; 8 doctests passed
• cargo clippy --all-targets: clean
• builds clean with and without --no-default-features
• public API byte-identical to docs/PUBLIC_API.txt (scripts/check-public-api.sh exits 0 with no diff) — SemVer-safe, no doc update required

The macros are private (no #[macro_export]) and expand to the same items that were written by hand, so the change is purely internal.

[jskoiz]

Review (2026-06-12). The red CI is just formatting: cargo fmt --all --check fails with a diff at src/ast.rs:1843 — the mapping_projection_iter! invocation for Iter<'a>. Both the ubuntu and windows "Rust checks" jobs die on that step (everything else, including MSRV and supply-chain audit, is green). Running cargo fmt --all and re-pushing should make the PR fully green; it's otherwise MERGEABLE against main.

Went through the macro expansions against the original hand-written bodies:

• forward_singleton_deserialize! / forward_visit_scalars! (src/with.rs) — method lists are complete and expansions match the originals exactly.
• mapping_projection_iter! (src/ast.rs) — all 7 projection-iterator invocations (including the multi-line IterMut closure) preserve the original next/size_hint/len/double-ended behavior.
• src/event_de/source.rs — scalar_key_at / scalar_key_at_in are confirmed unused on this branch (scalar_key_at was only called by scalar_key_at_in); removal is safe.

One observation worth a line in the PR description: for NestedValueSerializer, the numeric methods previously delegated through ValueSerializer.serialize_*(value) and now inline Value::from(value) directly via the macro. The result is identical (the delegate was itself just Value::from), but it's the one place the refactor changes the shape of the code rather than purely folding duplicates, so calling it out keeps the "no behavior change" claim precise.