This PR makes `arith::Analyzer` a first-class tvm-ffi object.

The implementation splits the previous concrete `Analyzer` class into:

- `AnalyzerObj`, the mutable object node that owns analyzer state,
sub-analyzers, caches, and bindings
- `Analyzer`, a reference-counted `ObjectRef` handle that can be passed
across the tvm-ffi boundary

This allows Python and C++ to share the same analyzer instance, so
bindings, constraints, and cached facts can persist across FFI calls.

Public APIs that accept an analyzer now use `const arith::Analyzer&`,
while internal helper APIs that only borrow the object continue to use
`AnalyzerObj*`.

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Co-authored-by: Ubospica <ubospica@gmail.com>