Phase out Relax-specific Id aliases

[tqchen]

Remove the Relax-specific Id indirection and use Var/DataflowVar object identity directly.

Type-changing rewrites now remap definitions, uses, and binding lookups coherently while preserving reflection, serialization, and the DataflowVar subtype. Existing tests are adjusted for the API change; no new test files or test cases are added.

Validation: the compiler and C++ tests build successfully; focused C++ coverage passes 4/4; the affected existing Python matrices pass 549 tests with 2 expected xfails; source censuses, diff checks, and applicable hooks pass.

[gemini-code-assist]

Code Review

This pull request simplifies variable identification in Relax by removing the Id class and instead identifying variables uniquely by their memory address. Consequently, VarNode now stores name_hint_ directly, and all mapping structures (such as var_remap_ and binding_table_) have been updated to use Var as keys instead of Id. Feedback on these changes highlights a potential performance bottleneck in the newly introduced RedirectVarRemapTargets helper, which performs a linear scan and introduces O(N^2) complexity; implementing a transitive lookup strategy is recommended to resolve this issue efficiently.

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