Move stream related code to streams package

[emil14]

Summary

• move the stream item type and supporting helpers from std/builtin into std/streams as streams.Item
• update standard library components, examples, and e2e fixtures to call the new streams and lists APIs
• teach the compiler desugarer to import the streams package when range syntax is used and refresh related docs

Testing

• go test ./internal/...
• go test ./... (fails: missing neva executable in PATH during e2e runs)

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https://chatgpt.com/codex/tasks/task_e_68fd27cf9cf4832d997a8b91928af682

[chatgpt-codex-connector]

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