Improve debugging user experience of `compiler/examples/fibonacci`

We will use this issue as a place holder of improving debug info and debugging experience on simple example.

Found that compiler also should be improved -- both spans and debug variable handling can be improved as well as debugger.

Look at the compiler/examples/fibonacci program to understand its structure. Examine the current span and debug variable handling in the compiler. The debugger's interaction with this example needs improvement. Start by running the example in the debugger to identify specific pain points in the debugging output and stepping experience.

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