Trait implementation splitting

[hanbings-agent]

feat: Introduce experimental impl shards syntax

This PR introduces an experimental impl shards syntax, allowing a trait implementation to be split into multiple impl items, each defining a single associated item.

Why:
To enable more granular organization of trait implementations, potentially across different parts of a module. This is a foundational step for exploring "impl shards" as requested by the user.

How it works:

1. New Syntax: impl Trait for Type fn method() { ... } is parsed under the #[feature(impl_shards)] gate.
2. Pre-expansion Merging: A new AST pass (rustc_ast_passes::impl_shards) merges these single-item impl shards into a single, conventional impl Trait for Type { ... } block. This occurs early in the compilation pipeline, before macro expansion and name resolution.
3. Scope: Merging is currently limited to impl shards within the same module that share identical impl headers (trait, self type, generics, where clause, modifiers).
4. Compatibility: Existing compiler checks for duplicate associated items and incomplete trait implementations are preserved, as they operate on the merged impl block.

Current Limitations:

• Shards generated via include!() or macros across different files/modules will not be merged, as the merging pass is module-local and pre-expansion. This means cross-module shards will still trigger existing coherence errors.

Tests:

• tests/ui/impl-shards/split.rs: Verifies successful compilation of split impls.
• tests/ui/impl-shards/feature-gate.rs: Ensures E0658 is emitted without the feature gate.
• tests/ui/impl-shards/duplicate.rs: Confirms E0201 for duplicate method definitions after merging.

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