Rust: Clean up some models

[geoffw0]

Clean up some models:

• moving model files into more appropriate directories.
• moving models into more appropriate model files.
• deleting duplicate models.

Shouldn't affect results (apart perhaps from lists of models used in tests).

[Copilot]

Pull Request Overview

This pull request reorganizes model definitions across the Rust standard library and framework files to improve code organization. The key changes involve moving models to more appropriate locations based on their module origins and adding support for the tokio-postgres and async-std frameworks.

• Added new model files for tokio-postgres and async-std frameworks
• Moved core::alloc::layout::Layout models from core.model.yml to alloc.model.yml
• Moved iterator chain/take models from tokio/io.model.yml to core.model.yml
• Reorganized string and arithmetic operation models between core and alloc files

Reviewed Changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rust/ql/lib/codeql/rust/frameworks/tokio/postgres.model.yml	New file adding SQL injection sinks and database sources for tokio-postgres client and row types
rust/ql/lib/codeql/rust/frameworks/tokio/io.model.yml	Removed misplaced iterator models that belong in core stdlib
rust/ql/lib/codeql/rust/frameworks/stdlib/core.model.yml	Added arithmetic operations and iterator models, removed Layout models moved to alloc, cleaned up string operation models
rust/ql/lib/codeql/rust/frameworks/stdlib/alloc.model.yml	Added Layout models from core, removed duplicate string operation models now in core
rust/ql/lib/codeql/rust/frameworks/asyncstd/net.model.yml	New file adding async-std network sources and I/O summaries

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[geoffw0]

Thanks for re-approving!