[MLIR] Add support for Tuples with integer keys

[rsetaluri]

See #1252.

Fixes this issue by prefixing tuple field keys with "_". This must be in sync between:

• The emission of struct types
• The emission of struct extraction

Note that struct creation does not need to be aware of this.

We use a common tuple_key_to_str() function (still specific to MLIR) to achieve this.

[leonardt]

Looks good, but I wonder if we could somehow encapsulate the retrieval of a safe index string into the type? E.g. add an API to tuple/product to get a "safe_index" for each field (where tuple would prepend the underscore). This would avoid having to deal with the Product vs Tuple check in the backend and allows for future extensibility (e.g. a user could subclass Tuple and define custom behavior, but they may need the ability to provide a "safe_index").