Add support for nested exception testing in Test/STM

[yogeshsajanikar]

Is your feature request related to a problem? Please describe.
Test/STM.hs contains a toy DSL which can be evaluated in a pure as well as the STM context. It is used to test the correctness of the io-sim STM implementation. The PR #16 implements the support for MonadCatch. However, the test code does not have support for nested exceptions. Adding a support for nested exception evaluation in Test/STM will make it a complete.

Describe the solution you'd like

• Add a support for multiple exception values (Currently ImmValue is treated as an exception type).
• Add another exception type (E.g. ImmValueException so that catch handler can do a type check).
• Make necessary changes to Expr generator and Expr evaluation.

Describe alternatives you've considered

• Write a test which runs in io-sim and io and has nested exception. However, it will be a unit test and will not test various scenarios when a test is generated through Expr generator.

Additional context
NA

Are you willing to implement it?

• Are you? 😃 - Already started working on it.