Refactor Transaction<T> to support cross-type STM variables by removing generic parameter

[engineering87]

Description

Currently, the Transaction<T> class is generic and constrained to handle STM variables of a single type T per transaction. This design limits the ability to perform atomic operations across multiple STM variables of different types within the same transaction.

Problem

• Transactions cannot span variables of different types simultaneously (e.g., mixing int and string STM variables in one atomic operation).
• This restricts practical use cases where shared state is composed of heterogeneous types.

Proposed solution

• Remove the generic parameter <T> from the Transaction class, making it non-generic.
• Adapt the internal data structures to hold STM variables of different types, possibly using a non-generic ISTMVariable interface or an object-based approach.
• Implement safe casting and type checking during read/write operations to maintain type safety at runtime.

Benefits

• Enables atomic transactions involving multiple STM variables of different types.
• Increases flexibility and expressiveness of STM operations.
• Aligns with common STM usage patterns where shared state is heterogeneous.

Considerations

• May introduce some overhead due to boxing/unboxing and runtime casts.
• Requires thorough testing to ensure type safety and performance are not adversely affected.